<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2025 Workiva-->
<!--r:ca2b7fcf-87dc-46e3-bfbc-0c0d8846cd39,g:5c0b25c7-5ace-45cc-b445-3ee05bf210ff-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.billiontoone.com/role/Cover" xlink:type="simple" xlink:href="blln-20250930.xsd#Cover"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/Cover" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_8a905003-fc29-4425-82f6-f9d49a4edb73" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_DocumentInformationTable_8a905003-fc29-4425-82f6-f9d49a4edb73" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_618cef4b-5322-4711-beeb-878cbd4e2eb6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_8a905003-fc29-4425-82f6-f9d49a4edb73" xlink:to="loc_us-gaap_StatementClassOfStockAxis_618cef4b-5322-4711-beeb-878cbd4e2eb6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_618cef4b-5322-4711-beeb-878cbd4e2eb6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_618cef4b-5322-4711-beeb-878cbd4e2eb6" xlink:to="loc_us-gaap_ClassOfStockDomain_618cef4b-5322-4711-beeb-878cbd4e2eb6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_c7a06f7e-96c7-488d-8791-ec17540b3f5a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_618cef4b-5322-4711-beeb-878cbd4e2eb6" xlink:to="loc_us-gaap_ClassOfStockDomain_c7a06f7e-96c7-488d-8791-ec17540b3f5a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_9fed9bad-f55e-4be1-974c-5d141db2bb8a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_c7a06f7e-96c7-488d-8791-ec17540b3f5a" xlink:to="loc_us-gaap_CommonClassAMember_9fed9bad-f55e-4be1-974c-5d141db2bb8a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_5179ba54-912f-4692-abbe-5c3afd4d960f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_c7a06f7e-96c7-488d-8791-ec17540b3f5a" xlink:to="loc_us-gaap_CommonClassBMember_5179ba54-912f-4692-abbe-5c3afd4d960f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_147f1b0d-8f3a-4a03-b025-69a84de41080" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_DocumentType_147f1b0d-8f3a-4a03-b025-69a84de41080" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_28cb82da-539b-4aa7-9023-1ce21c9506cc" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_DocumentQuarterlyReport_28cb82da-539b-4aa7-9023-1ce21c9506cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_446c49bd-5d45-4638-bf2b-1fe66e7a9ed9" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_DocumentTransitionReport_446c49bd-5d45-4638-bf2b-1fe66e7a9ed9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_d34b56ab-d998-444c-acac-a098d4b5e528" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_DocumentPeriodEndDate_d34b56ab-d998-444c-acac-a098d4b5e528" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_c0a556e1-31ae-483c-9ca1-8377f58b9d83" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityFileNumber_c0a556e1-31ae-483c-9ca1-8377f58b9d83" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_cad831f9-4d72-484e-9970-ab9f684b5e44" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityRegistrantName_cad831f9-4d72-484e-9970-ab9f684b5e44" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_e1a02061-48fd-4377-bcb9-3adf5d78b53d" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityIncorporationStateCountryCode_e1a02061-48fd-4377-bcb9-3adf5d78b53d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_020eb61f-1a5c-4b71-a2bc-6382d298f79a" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityTaxIdentificationNumber_020eb61f-1a5c-4b71-a2bc-6382d298f79a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_5afbccb0-513b-4e40-a1d0-70cfa40690de" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityAddressAddressLine1_5afbccb0-513b-4e40-a1d0-70cfa40690de" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_1034870b-1297-499e-997a-904b80a1345e" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityAddressCityOrTown_1034870b-1297-499e-997a-904b80a1345e" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_6c1f6bb4-e563-4300-aa91-d0c3cfbed16a" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityAddressStateOrProvince_6c1f6bb4-e563-4300-aa91-d0c3cfbed16a" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_289e7bf8-4ca4-4953-8ac6-bd9fcbd49a7b" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityAddressPostalZipCode_289e7bf8-4ca4-4953-8ac6-bd9fcbd49a7b" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_1402e200-0699-4384-b4da-f7e554fa953f" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_CityAreaCode_1402e200-0699-4384-b4da-f7e554fa953f" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_3e0f0b5c-b178-4999-a4fd-773389e61154" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_LocalPhoneNumber_3e0f0b5c-b178-4999-a4fd-773389e61154" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_a9aa0643-0716-44a4-b702-e45146566e48" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_Security12bTitle_a9aa0643-0716-44a4-b702-e45146566e48" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_4a6967a5-33d4-4ea0-b9aa-9937f37bbed4" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_TradingSymbol_4a6967a5-33d4-4ea0-b9aa-9937f37bbed4" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_099b68b1-1316-46fb-940a-e31fe6020328" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_SecurityExchangeName_099b68b1-1316-46fb-940a-e31fe6020328" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_16bde24e-3fc5-4acb-95bc-e838d816ef77" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityCurrentReportingStatus_16bde24e-3fc5-4acb-95bc-e838d816ef77" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_7c4b5fd0-2dd8-4dce-a7c8-88d4b9123a1c" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityInteractiveDataCurrent_7c4b5fd0-2dd8-4dce-a7c8-88d4b9123a1c" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_09301de5-b40f-471d-936e-46fcdf4a9f73" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityFilerCategory_09301de5-b40f-471d-936e-46fcdf4a9f73" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_d15c6333-e621-42a8-8097-e8299b30f8fc" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntitySmallBusiness_d15c6333-e621-42a8-8097-e8299b30f8fc" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_e130db3e-f6ef-45e0-b2a1-be943397e487" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityEmergingGrowthCompany_e130db3e-f6ef-45e0-b2a1-be943397e487" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityExTransitionPeriod_70fb5f1a-e93a-405d-b6e2-7c610e187471" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityExTransitionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityExTransitionPeriod_70fb5f1a-e93a-405d-b6e2-7c610e187471" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_98b0adac-f848-4bc6-a63a-18965a66e9a8" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityShellCompany_98b0adac-f848-4bc6-a63a-18965a66e9a8" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_73d8ab8d-85ca-4f2d-b988-a10d8e52767a" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_73d8ab8d-85ca-4f2d-b988-a10d8e52767a" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_06502953-9518-45dc-a10c-8737226789a6" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_EntityCentralIndexKey_06502953-9518-45dc-a10c-8737226789a6" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_4ca0c12d-c9f1-47a9-bc42-a3d8149dbe19" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_CurrentFiscalYearEndDate_4ca0c12d-c9f1-47a9-bc42-a3d8149dbe19" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_16906703-e25d-4978-8c4b-1a55facdf0dd" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_DocumentFiscalYearFocus_16906703-e25d-4978-8c4b-1a55facdf0dd" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_75749ea5-0242-4dda-9ff9-322236036961" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_DocumentFiscalPeriodFocus_75749ea5-0242-4dda-9ff9-322236036961" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_dfdc7a3e-42b4-48a5-a6c9-b1a243169734" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4ceb0d01-092f-4ae8-ab59-9d6f57c92973" xlink:to="loc_dei_AmendmentFlag_dfdc7a3e-42b4-48a5-a6c9-b1a243169734" xlink:type="arc" order="29"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/StatementsofRedeemableConvertiblePreferredStockandStockholdersDeficit" xlink:type="simple" xlink:href="blln-20250930.xsd#StatementsofRedeemableConvertiblePreferredStockandStockholdersDeficit"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/StatementsofRedeemableConvertiblePreferredStockandStockholdersDeficit" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_b4aa7cb8-17d3-4840-99ee-c498ca1fb375" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_71e5e08f-d9a1-4039-b3c9-4d469e37dc4f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_b4aa7cb8-17d3-4840-99ee-c498ca1fb375" xlink:to="loc_us-gaap_StatementTable_71e5e08f-d9a1-4039-b3c9-4d469e37dc4f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_16f43092-c9c0-486e-87f4-f0c45aad27db" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_71e5e08f-d9a1-4039-b3c9-4d469e37dc4f" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_16f43092-c9c0-486e-87f4-f0c45aad27db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_16f43092-c9c0-486e-87f4-f0c45aad27db_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_16f43092-c9c0-486e-87f4-f0c45aad27db" xlink:to="loc_us-gaap_EquityComponentDomain_16f43092-c9c0-486e-87f4-f0c45aad27db_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_6368cf08-b7fd-4883-9dc9-054329fca123" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_16f43092-c9c0-486e-87f4-f0c45aad27db" xlink:to="loc_us-gaap_EquityComponentDomain_6368cf08-b7fd-4883-9dc9-054329fca123" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_b717b5ec-741a-40cc-a658-f5137bbbf960" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_6368cf08-b7fd-4883-9dc9-054329fca123" xlink:to="loc_us-gaap_CommonStockMember_b717b5ec-741a-40cc-a658-f5137bbbf960" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_a4d1a2a7-c739-4eea-85d9-78eb0389d6a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_6368cf08-b7fd-4883-9dc9-054329fca123" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_a4d1a2a7-c739-4eea-85d9-78eb0389d6a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_28a040dc-5f79-46dd-ac4f-6a312805cc76" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_6368cf08-b7fd-4883-9dc9-054329fca123" xlink:to="loc_us-gaap_RetainedEarningsMember_28a040dc-5f79-46dd-ac4f-6a312805cc76" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_19fe26e1-386f-4d67-8d22-acc9e43311af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_71e5e08f-d9a1-4039-b3c9-4d469e37dc4f" xlink:to="loc_us-gaap_StatementClassOfStockAxis_19fe26e1-386f-4d67-8d22-acc9e43311af" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_19fe26e1-386f-4d67-8d22-acc9e43311af_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_19fe26e1-386f-4d67-8d22-acc9e43311af" xlink:to="loc_us-gaap_ClassOfStockDomain_19fe26e1-386f-4d67-8d22-acc9e43311af_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_755a3bc2-a86a-48dd-af45-9c6fa22e1dc5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_19fe26e1-386f-4d67-8d22-acc9e43311af" xlink:to="loc_us-gaap_ClassOfStockDomain_755a3bc2-a86a-48dd-af45-9c6fa22e1dc5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesC1RedeemableConvertiblePreferredStockMember_e247363d-dcc3-4aac-a730-58aee7f1e615" xlink:href="blln-20250930.xsd#blln_SeriesC1RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_755a3bc2-a86a-48dd-af45-9c6fa22e1dc5" xlink:to="loc_blln_SeriesC1RedeemableConvertiblePreferredStockMember_e247363d-dcc3-4aac-a730-58aee7f1e615" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesDRedeemableConvertiblePreferredStockMember_a72c40db-a20b-4f8f-b41c-469c069a9b41" xlink:href="blln-20250930.xsd#blln_SeriesDRedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_755a3bc2-a86a-48dd-af45-9c6fa22e1dc5" xlink:to="loc_blln_SeriesDRedeemableConvertiblePreferredStockMember_a72c40db-a20b-4f8f-b41c-469c069a9b41" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_ede2973e-3517-4ec5-828c-b6689a5c8a3a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInTemporaryEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_b4aa7cb8-17d3-4840-99ee-c498ca1fb375" xlink:to="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_ede2973e-3517-4ec5-828c-b6689a5c8a3a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_fd14890b-e806-4b02-8b8e-1c3fcc149987" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquitySharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_ede2973e-3517-4ec5-828c-b6689a5c8a3a" xlink:to="loc_us-gaap_TemporaryEquitySharesOutstanding_fd14890b-e806-4b02-8b8e-1c3fcc149987" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_4427c90a-ecf5-40de-81ac-e4e31b9d582f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_ede2973e-3517-4ec5-828c-b6689a5c8a3a" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_4427c90a-ecf5-40de-81ac-e4e31b9d582f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues_b7df9cf1-2576-4f06-b0e6-dc360c357db1" xlink:href="blln-20250930.xsd#blln_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_ede2973e-3517-4ec5-828c-b6689a5c8a3a" xlink:to="loc_blln_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues_b7df9cf1-2576-4f06-b0e6-dc360c357db1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_TemporaryEquityStockIssuedDuringPeriodValueNetOfIssuanceCostsNewIssues_e73d37f6-8b4c-4e8a-8282-c7a49637d754" xlink:href="blln-20250930.xsd#blln_TemporaryEquityStockIssuedDuringPeriodValueNetOfIssuanceCostsNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_ede2973e-3517-4ec5-828c-b6689a5c8a3a" xlink:to="loc_blln_TemporaryEquityStockIssuedDuringPeriodValueNetOfIssuanceCostsNewIssues_e73d37f6-8b4c-4e8a-8282-c7a49637d754" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues_8d15e983-adf4-4bb1-9b17-b55dd5d431b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_ede2973e-3517-4ec5-828c-b6689a5c8a3a" xlink:to="loc_us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues_8d15e983-adf4-4bb1-9b17-b55dd5d431b9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_d4447dad-d012-46e9-abac-480ab17bb760" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquitySharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_22ddf87d-4dbd-440a-8e70-8dea049963d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_bcde0cd9-8143-4a63-ae1a-1cff875c73e2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_b4aa7cb8-17d3-4840-99ee-c498ca1fb375" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_bcde0cd9-8143-4a63-ae1a-1cff875c73e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_1233d996-c320-4249-8d67-1205752d8425" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_bcde0cd9-8143-4a63-ae1a-1cff875c73e2" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_1233d996-c320-4249-8d67-1205752d8425" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_f18e5728-8a15-4938-a7d0-6ef8363dd7d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_bcde0cd9-8143-4a63-ae1a-1cff875c73e2" xlink:to="loc_us-gaap_StockholdersEquity_f18e5728-8a15-4938-a7d0-6ef8363dd7d1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_9968dc3c-d7ea-44d7-836c-00fbe4fc6004" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_bcde0cd9-8143-4a63-ae1a-1cff875c73e2" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_9968dc3c-d7ea-44d7-836c-00fbe4fc6004" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_c56bd1dc-67e1-43f7-899b-de9fac7c76a5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_bcde0cd9-8143-4a63-ae1a-1cff875c73e2" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_c56bd1dc-67e1-43f7-899b-de9fac7c76a5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_de03c390-5996-4001-8d97-08546414a284" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_bcde0cd9-8143-4a63-ae1a-1cff875c73e2" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_de03c390-5996-4001-8d97-08546414a284" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_2bbe3923-acc6-46e1-ab70-3f6b64a6318b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_bcde0cd9-8143-4a63-ae1a-1cff875c73e2" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_2bbe3923-acc6-46e1-ab70-3f6b64a6318b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_9d7ca147-ef28-4dd6-9628-d767519b17be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_bcde0cd9-8143-4a63-ae1a-1cff875c73e2" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_9d7ca147-ef28-4dd6-9628-d767519b17be" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_AdjustmentToAdditionalPaidInCapitalVestingOfEarlyExercises_f8f1f4d0-d75b-482a-a14e-69d363b20639" xlink:href="blln-20250930.xsd#blln_AdjustmentToAdditionalPaidInCapitalVestingOfEarlyExercises"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_bcde0cd9-8143-4a63-ae1a-1cff875c73e2" xlink:to="loc_blln_AdjustmentToAdditionalPaidInCapitalVestingOfEarlyExercises_f8f1f4d0-d75b-482a-a14e-69d363b20639" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_1792cb28-3fc9-4a2d-ab17-81979bf7abf6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_bcde0cd9-8143-4a63-ae1a-1cff875c73e2" xlink:to="loc_us-gaap_NetIncomeLoss_1792cb28-3fc9-4a2d-ab17-81979bf7abf6" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_efbcc529-9f45-4f21-b828-001d3628151e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_aa56417a-0859-42d3-afa2-fc65fdaa3fc2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfStockIssuanceCosts_0cdac2ce-6663-40d7-a45d-9797fc7dfaf3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfStockIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_b4aa7cb8-17d3-4840-99ee-c498ca1fb375" xlink:to="loc_us-gaap_PaymentsOfStockIssuanceCosts_0cdac2ce-6663-40d7-a45d-9797fc7dfaf3" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/SummaryofSignificantAccountingPoliciesDisaggregationofRevenueDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#SummaryofSignificantAccountingPoliciesDisaggregationofRevenueDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/SummaryofSignificantAccountingPoliciesDisaggregationofRevenueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_f6a3f82e-a3ed-4ee7-a4b5-9c5937e21fff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_d808f768-84f6-4f80-9722-6517faf81d06" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_f6a3f82e-a3ed-4ee7-a4b5-9c5937e21fff" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_d808f768-84f6-4f80-9722-6517faf81d06" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_a5f9b2d9-46b3-48ae-9a2f-3b095390bfe0" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_d808f768-84f6-4f80-9722-6517faf81d06" xlink:to="loc_srt_ProductOrServiceAxis_a5f9b2d9-46b3-48ae-9a2f-3b095390bfe0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_a5f9b2d9-46b3-48ae-9a2f-3b095390bfe0_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_a5f9b2d9-46b3-48ae-9a2f-3b095390bfe0" xlink:to="loc_srt_ProductsAndServicesDomain_a5f9b2d9-46b3-48ae-9a2f-3b095390bfe0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_9abf542a-18ce-4324-903e-83342fcb4e4f" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_a5f9b2d9-46b3-48ae-9a2f-3b095390bfe0" xlink:to="loc_srt_ProductsAndServicesDomain_9abf542a-18ce-4324-903e-83342fcb4e4f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_PrenatalMember_6b9e74eb-3dcd-4be2-b717-f3192cfdf2c5" xlink:href="blln-20250930.xsd#blln_PrenatalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_9abf542a-18ce-4324-903e-83342fcb4e4f" xlink:to="loc_blln_PrenatalMember_6b9e74eb-3dcd-4be2-b717-f3192cfdf2c5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_OncologyMember_422d972a-f4b5-481b-b4f3-c1c5657b7959" xlink:href="blln-20250930.xsd#blln_OncologyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_9abf542a-18ce-4324-903e-83342fcb4e4f" xlink:to="loc_blln_OncologyMember_422d972a-f4b5-481b-b4f3-c1c5657b7959" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_ClinicalTrialSupportAndOtherServicesMember_7f4ba1ad-06ab-4ded-a178-7dada56d0fb2" xlink:href="blln-20250930.xsd#blln_ClinicalTrialSupportAndOtherServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_9abf542a-18ce-4324-903e-83342fcb4e4f" xlink:to="loc_blln_ClinicalTrialSupportAndOtherServicesMember_7f4ba1ad-06ab-4ded-a178-7dada56d0fb2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_efc44c25-9251-4456-bfa6-d84415a553cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_f6a3f82e-a3ed-4ee7-a4b5-9c5937e21fff" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_efc44c25-9251-4456-bfa6-d84415a553cc" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/FairValueMeasurementsSummaryofFairValueMeasurementofAssetsandLiabilitiesDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#FairValueMeasurementsSummaryofFairValueMeasurementofAssetsandLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/FairValueMeasurementsSummaryofFairValueMeasurementofAssetsandLiabilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5916a234-099a-40cc-8a78-f94abcbe3722" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_f0caf3ee-bd26-465f-a6b3-0044996a9cf8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5916a234-099a-40cc-8a78-f94abcbe3722" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_f0caf3ee-bd26-465f-a6b3-0044996a9cf8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_e4453dfc-8866-43df-a22c-48eeab7e1776" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_f0caf3ee-bd26-465f-a6b3-0044996a9cf8" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_e4453dfc-8866-43df-a22c-48eeab7e1776" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_e4453dfc-8866-43df-a22c-48eeab7e1776_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_e4453dfc-8866-43df-a22c-48eeab7e1776" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_e4453dfc-8866-43df-a22c-48eeab7e1776_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2953391f-72f2-4632-bb75-2b37c0280ac0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_e4453dfc-8866-43df-a22c-48eeab7e1776" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2953391f-72f2-4632-bb75-2b37c0280ac0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_74b04bf0-2950-4050-8c00-e29b31d7b563" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2953391f-72f2-4632-bb75-2b37c0280ac0" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_74b04bf0-2950-4050-8c00-e29b31d7b563" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_195f294f-15e9-43ca-9e24-fbf4b600945a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2953391f-72f2-4632-bb75-2b37c0280ac0" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_195f294f-15e9-43ca-9e24-fbf4b600945a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_7819a4ea-611f-4c9a-8a57-7a10116ee159" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2953391f-72f2-4632-bb75-2b37c0280ac0" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_7819a4ea-611f-4c9a-8a57-7a10116ee159" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_a2014f9a-5e45-4a38-81af-f11345db5c4f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_f0caf3ee-bd26-465f-a6b3-0044996a9cf8" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_a2014f9a-5e45-4a38-81af-f11345db5c4f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_a2014f9a-5e45-4a38-81af-f11345db5c4f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_a2014f9a-5e45-4a38-81af-f11345db5c4f" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_a2014f9a-5e45-4a38-81af-f11345db5c4f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_2e46315a-8f01-4834-988b-9a1fa0db66e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_a2014f9a-5e45-4a38-81af-f11345db5c4f" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_2e46315a-8f01-4834-988b-9a1fa0db66e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_6b90bf24-c02f-494b-8c7c-fd29c1cf9006" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_2e46315a-8f01-4834-988b-9a1fa0db66e0" xlink:to="loc_us-gaap_MoneyMarketFundsMember_6b90bf24-c02f-494b-8c7c-fd29c1cf9006" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_a31b8a11-0add-4da7-acf8-cc25bdbf96d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_f0caf3ee-bd26-465f-a6b3-0044996a9cf8" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_a31b8a11-0add-4da7-acf8-cc25bdbf96d4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_a31b8a11-0add-4da7-acf8-cc25bdbf96d4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_a31b8a11-0add-4da7-acf8-cc25bdbf96d4" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_a31b8a11-0add-4da7-acf8-cc25bdbf96d4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_585c614b-3070-430c-a7a3-9caa99049d95" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_a31b8a11-0add-4da7-acf8-cc25bdbf96d4" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_585c614b-3070-430c-a7a3-9caa99049d95" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_RedeemableConvertiblePreferredStockWarrantsMember_e3a068da-ca57-4360-8f59-133e0e459e37" xlink:href="blln-20250930.xsd#blln_RedeemableConvertiblePreferredStockWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_585c614b-3070-430c-a7a3-9caa99049d95" xlink:to="loc_blln_RedeemableConvertiblePreferredStockWarrantsMember_e3a068da-ca57-4360-8f59-133e0e459e37" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_CommonStockWarrantsMember_f612e5d4-0872-4c7e-afaa-0c52ae99dfac" xlink:href="blln-20250930.xsd#blln_CommonStockWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_585c614b-3070-430c-a7a3-9caa99049d95" xlink:to="loc_blln_CommonStockWarrantsMember_f612e5d4-0872-4c7e-afaa-0c52ae99dfac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosureAbstract_f266a847-88a0-44e5-8504-c8a25bdce67f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5916a234-099a-40cc-8a78-f94abcbe3722" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_f266a847-88a0-44e5-8504-c8a25bdce67f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_3b42fb24-c18b-4a34-970e-2631a5f819c0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_f266a847-88a0-44e5-8504-c8a25bdce67f" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_3b42fb24-c18b-4a34-970e-2631a5f819c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_450ab916-1499-495e-9b12-2afff40cb7d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_f266a847-88a0-44e5-8504-c8a25bdce67f" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_450ab916-1499-495e-9b12-2afff40cb7d7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_bc8f5d22-5d8a-41ee-a0c5-a4c81abe94b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_5916a234-099a-40cc-8a78-f94abcbe3722" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_bc8f5d22-5d8a-41ee-a0c5-a4c81abe94b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_WarrantsFairValueDisclosure_eaae8305-0973-455b-b975-eb6a18f62ab3" xlink:href="blln-20250930.xsd#blln_WarrantsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_bc8f5d22-5d8a-41ee-a0c5-a4c81abe94b7" xlink:to="loc_blln_WarrantsFairValueDisclosure_eaae8305-0973-455b-b975-eb6a18f62ab3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansPayableFairValueDisclosure_47744110-e31a-451a-8e10-23526b27a26b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoansPayableFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_bc8f5d22-5d8a-41ee-a0c5-a4c81abe94b7" xlink:to="loc_us-gaap_LoansPayableFairValueDisclosure_47744110-e31a-451a-8e10-23526b27a26b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_4e660ced-a33e-49a0-b740-86f256229592" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_bc8f5d22-5d8a-41ee-a0c5-a4c81abe94b7" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_4e660ced-a33e-49a0-b740-86f256229592" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/FairValueMeasurementsSummaryofSignificantAssumptionsUsedinPreparingtheOptionPricingModelDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#FairValueMeasurementsSummaryofSignificantAssumptionsUsedinPreparingtheOptionPricingModelDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/FairValueMeasurementsSummaryofSignificantAssumptionsUsedinPreparingtheOptionPricingModelDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_cfe73ba5-b38c-4b65-877a-be4c594c724a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_975ef759-51cb-4de7-90d7-c0c1dd472b66" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_cfe73ba5-b38c-4b65-877a-be4c594c724a" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_975ef759-51cb-4de7-90d7-c0c1dd472b66" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_4e2c94c7-b587-478b-9f25-ff4d850bf5dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_975ef759-51cb-4de7-90d7-c0c1dd472b66" xlink:to="loc_us-gaap_StatementClassOfStockAxis_4e2c94c7-b587-478b-9f25-ff4d850bf5dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_4e2c94c7-b587-478b-9f25-ff4d850bf5dd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_4e2c94c7-b587-478b-9f25-ff4d850bf5dd" xlink:to="loc_us-gaap_ClassOfStockDomain_4e2c94c7-b587-478b-9f25-ff4d850bf5dd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_893f8d7f-d4b5-4287-ac0d-e3a9a3eec950" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_4e2c94c7-b587-478b-9f25-ff4d850bf5dd" xlink:to="loc_us-gaap_ClassOfStockDomain_893f8d7f-d4b5-4287-ac0d-e3a9a3eec950" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RedeemableConvertiblePreferredStockMember_c2754928-47c2-4a2f-b040-e51800acbf77" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_893f8d7f-d4b5-4287-ac0d-e3a9a3eec950" xlink:to="loc_us-gaap_RedeemableConvertiblePreferredStockMember_c2754928-47c2-4a2f-b040-e51800acbf77" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_2c2bd755-e063-4797-aeb1-0651865ebd4c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_975ef759-51cb-4de7-90d7-c0c1dd472b66" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_2c2bd755-e063-4797-aeb1-0651865ebd4c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_2c2bd755-e063-4797-aeb1-0651865ebd4c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_2c2bd755-e063-4797-aeb1-0651865ebd4c" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_2c2bd755-e063-4797-aeb1-0651865ebd4c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_2699512d-73ef-4d5b-9850-2604be30b5ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_2c2bd755-e063-4797-aeb1-0651865ebd4c" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_2699512d-73ef-4d5b-9850-2604be30b5ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputSharePriceMember_901143a2-d531-45a4-93b3-1f62ba6373d5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputSharePriceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_2699512d-73ef-4d5b-9850-2604be30b5ae" xlink:to="loc_us-gaap_MeasurementInputSharePriceMember_901143a2-d531-45a4-93b3-1f62ba6373d5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExercisePriceMember_299bc8cc-6118-4c56-8bc9-b530a99dc628" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputExercisePriceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_2699512d-73ef-4d5b-9850-2604be30b5ae" xlink:to="loc_us-gaap_MeasurementInputExercisePriceMember_299bc8cc-6118-4c56-8bc9-b530a99dc628" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputPriceVolatilityMember_cd77897d-6441-4108-a3a5-97b33561a0b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputPriceVolatilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_2699512d-73ef-4d5b-9850-2604be30b5ae" xlink:to="loc_us-gaap_MeasurementInputPriceVolatilityMember_cd77897d-6441-4108-a3a5-97b33561a0b7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedTermMember_6eb52985-3ddf-4f8f-b624-eef6a624e45d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputExpectedTermMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_2699512d-73ef-4d5b-9850-2604be30b5ae" xlink:to="loc_us-gaap_MeasurementInputExpectedTermMember_6eb52985-3ddf-4f8f-b624-eef6a624e45d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_e3978693-4ead-460c-b7be-a9be2b93fd1d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_2699512d-73ef-4d5b-9850-2604be30b5ae" xlink:to="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_e3978693-4ead-460c-b7be-a9be2b93fd1d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedDividendRateMember_f638c034-1158-4e12-9d31-4ec82a0024d5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputExpectedDividendRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_2699512d-73ef-4d5b-9850-2604be30b5ae" xlink:to="loc_us-gaap_MeasurementInputExpectedDividendRateMember_f638c034-1158-4e12-9d31-4ec82a0024d5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_MeasurementInputInterestRateMember_6f03363d-0402-45ca-a1dd-73fb766b4481" xlink:href="blln-20250930.xsd#blln_MeasurementInputInterestRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_2699512d-73ef-4d5b-9850-2604be30b5ae" xlink:to="loc_blln_MeasurementInputInterestRateMember_6f03363d-0402-45ca-a1dd-73fb766b4481" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputDiscountRateMember_6d4cf373-ff09-46c3-a7f8-6c59904d4fed" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputDiscountRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_2699512d-73ef-4d5b-9850-2604be30b5ae" xlink:to="loc_us-gaap_MeasurementInputDiscountRateMember_6d4cf373-ff09-46c3-a7f8-6c59904d4fed" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_MeasurementInputExpectedTerm2Member_be680cbf-15c6-4d6e-9207-bad42e65b332" xlink:href="blln-20250930.xsd#blln_MeasurementInputExpectedTerm2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_2699512d-73ef-4d5b-9850-2604be30b5ae" xlink:to="loc_blln_MeasurementInputExpectedTerm2Member_be680cbf-15c6-4d6e-9207-bad42e65b332" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_afd3239e-dcc7-4286-955b-cc5f555ee241" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_975ef759-51cb-4de7-90d7-c0c1dd472b66" xlink:to="loc_srt_RangeAxis_afd3239e-dcc7-4286-955b-cc5f555ee241" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_afd3239e-dcc7-4286-955b-cc5f555ee241_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_afd3239e-dcc7-4286-955b-cc5f555ee241" xlink:to="loc_srt_RangeMember_afd3239e-dcc7-4286-955b-cc5f555ee241_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_dfbb7f8b-b6a1-4f27-aabe-b037c998c61c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_afd3239e-dcc7-4286-955b-cc5f555ee241" xlink:to="loc_srt_RangeMember_dfbb7f8b-b6a1-4f27-aabe-b037c998c61c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_329f2e11-f7d5-44b7-a688-4d4747708922" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_dfbb7f8b-b6a1-4f27-aabe-b037c998c61c" xlink:to="loc_srt_MinimumMember_329f2e11-f7d5-44b7-a688-4d4747708922" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_e9d1a39d-4a7b-4b66-9e10-c2d428448715" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_dfbb7f8b-b6a1-4f27-aabe-b037c998c61c" xlink:to="loc_srt_MaximumMember_e9d1a39d-4a7b-4b66-9e10-c2d428448715" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_44fd18cd-a055-418f-97e6-18f314a5ea8e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByLiabilityClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_975ef759-51cb-4de7-90d7-c0c1dd472b66" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_44fd18cd-a055-418f-97e6-18f314a5ea8e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_44fd18cd-a055-418f-97e6-18f314a5ea8e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_44fd18cd-a055-418f-97e6-18f314a5ea8e" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_44fd18cd-a055-418f-97e6-18f314a5ea8e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_72289c98-2523-4668-bc5f-ad14cd50584f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_44fd18cd-a055-418f-97e6-18f314a5ea8e" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_72289c98-2523-4668-bc5f-ad14cd50584f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_672f8e10-800c-4a6d-8d82-96201b31191a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_72289c98-2523-4668-bc5f-ad14cd50584f" xlink:to="loc_us-gaap_ConvertibleDebtMember_672f8e10-800c-4a6d-8d82-96201b31191a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_TermLoanMember_ec3ec585-1dc7-4341-b89c-a40318ff5d06" xlink:href="blln-20250930.xsd#blln_TermLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_72289c98-2523-4668-bc5f-ad14cd50584f" xlink:to="loc_blln_TermLoanMember_ec3ec585-1dc7-4341-b89c-a40318ff5d06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_e8076c66-9b4a-4e44-ad46-3120833aca7e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_975ef759-51cb-4de7-90d7-c0c1dd472b66" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_e8076c66-9b4a-4e44-ad46-3120833aca7e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_e8076c66-9b4a-4e44-ad46-3120833aca7e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_e8076c66-9b4a-4e44-ad46-3120833aca7e" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_e8076c66-9b4a-4e44-ad46-3120833aca7e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_63bad64e-6eca-4354-827a-a0fc48b50f1e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_e8076c66-9b4a-4e44-ad46-3120833aca7e" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_63bad64e-6eca-4354-827a-a0fc48b50f1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_CommonStockWarrantsMember_f2f1c22f-5980-4ad1-b6b0-34675ec223a2" xlink:href="blln-20250930.xsd#blln_CommonStockWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_63bad64e-6eca-4354-827a-a0fc48b50f1e" xlink:to="loc_blln_CommonStockWarrantsMember_f2f1c22f-5980-4ad1-b6b0-34675ec223a2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_a0aaf35c-0121-426a-a94e-6df409c49409" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstandingMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_cfe73ba5-b38c-4b65-877a-be4c594c724a" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_a0aaf35c-0121-426a-a94e-6df409c49409" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentMeasurementInput_748ef167-2ea6-46e9-9384-4b90cff71041" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_cfe73ba5-b38c-4b65-877a-be4c594c724a" xlink:to="loc_us-gaap_DebtInstrumentMeasurementInput_748ef167-2ea6-46e9-9384-4b90cff71041" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_76de1dda-ae17-4b9d-b2b0-354eaf5a08c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_cfe73ba5-b38c-4b65-877a-be4c594c724a" xlink:to="loc_us-gaap_LongTermDebtFairValue_76de1dda-ae17-4b9d-b2b0-354eaf5a08c6" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/FairValueMeasurementsSummaryofChangesintheFairValueofLevel3FinancialInstrumentsDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#FairValueMeasurementsSummaryofChangesintheFairValueofLevel3FinancialInstrumentsDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/FairValueMeasurementsSummaryofChangesintheFairValueofLevel3FinancialInstrumentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_0aea7908-bcbe-42ea-bf1e-b1b248df8482" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_12eff54f-aaf9-4b30-8462-533e51783df0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_0aea7908-bcbe-42ea-bf1e-b1b248df8482" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_12eff54f-aaf9-4b30-8462-533e51783df0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByLiabilityClassAxis_a27368c6-f52d-455d-834c-ce6cea3042ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByLiabilityClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_12eff54f-aaf9-4b30-8462-533e51783df0" xlink:to="loc_us-gaap_FairValueByLiabilityClassAxis_a27368c6-f52d-455d-834c-ce6cea3042ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_a27368c6-f52d-455d-834c-ce6cea3042ae_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_a27368c6-f52d-455d-834c-ce6cea3042ae" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_a27368c6-f52d-455d-834c-ce6cea3042ae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_5cb39fce-a559-46d3-ab5d-78a04eaac193" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByLiabilityClassAxis_a27368c6-f52d-455d-834c-ce6cea3042ae" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_5cb39fce-a559-46d3-ab5d-78a04eaac193" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_CommonStockWarrantsMember_389ffb6f-7db2-4ce6-b2cc-6762106fe025" xlink:href="blln-20250930.xsd#blln_CommonStockWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_5cb39fce-a559-46d3-ab5d-78a04eaac193" xlink:to="loc_blln_CommonStockWarrantsMember_389ffb6f-7db2-4ce6-b2cc-6762106fe025" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_RedeemableConvertiblePreferredStockWarrantsMember_933494f7-77c9-4ac2-926d-24bb4fd58547" xlink:href="blln-20250930.xsd#blln_RedeemableConvertiblePreferredStockWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_5cb39fce-a559-46d3-ab5d-78a04eaac193" xlink:to="loc_blln_RedeemableConvertiblePreferredStockWarrantsMember_933494f7-77c9-4ac2-926d-24bb4fd58547" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_e89f11db-41dd-464a-ad34-3cdc0d4a27fa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_5cb39fce-a559-46d3-ab5d-78a04eaac193" xlink:to="loc_us-gaap_ConvertibleDebtMember_e89f11db-41dd-464a-ad34-3cdc0d4a27fa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_TermLoanMember_e0d2ab73-d7c3-41d4-9b13-70c1a0ff2405" xlink:href="blln-20250930.xsd#blln_TermLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain_5cb39fce-a559-46d3-ab5d-78a04eaac193" xlink:to="loc_blln_TermLoanMember_e0d2ab73-d7c3-41d4-9b13-70c1a0ff2405" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_87b79edb-b305-4490-a2db-91048d926003" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_0aea7908-bcbe-42ea-bf1e-b1b248df8482" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_87b79edb-b305-4490-a2db-91048d926003" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_b286937d-3230-4c3a-8c9e-0128684588b2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_87b79edb-b305-4490-a2db-91048d926003" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_b286937d-3230-4c3a-8c9e-0128684588b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_d0bf77f4-f145-43c6-879a-a2b30ecadb84" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_87b79edb-b305-4490-a2db-91048d926003" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_d0bf77f4-f145-43c6-879a-a2b30ecadb84" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainOnExtinguishment_44466672-08d6-44af-be62-0a44be0e7b4d" xlink:href="blln-20250930.xsd#blln_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainOnExtinguishment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_87b79edb-b305-4490-a2db-91048d926003" xlink:to="loc_blln_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainOnExtinguishment_44466672-08d6-44af-be62-0a44be0e7b4d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_17837d8d-f2f0-4e94-aa34-0693aecb537f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_87b79edb-b305-4490-a2db-91048d926003" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_17837d8d-f2f0-4e94-aa34-0693aecb537f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_923cbc16-df2b-445b-8111-bacad69d43af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_87b79edb-b305-4490-a2db-91048d926003" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_923cbc16-df2b-445b-8111-bacad69d43af" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_869803a6-b7c7-46c5-b685-5ee2bb5040fc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/RedeemableConvertiblePreferredStockDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#RedeemableConvertiblePreferredStockDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/RedeemableConvertiblePreferredStockDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityLineItems_5c696c04-91ac-434c-b5aa-d1a3420e0be5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityByClassOfStockTable_a968a259-e987-4d2e-96bf-d29e55d6b157" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityByClassOfStockTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_TemporaryEquityLineItems_5c696c04-91ac-434c-b5aa-d1a3420e0be5" xlink:to="loc_us-gaap_TemporaryEquityByClassOfStockTable_a968a259-e987-4d2e-96bf-d29e55d6b157" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_9ab2679b-2d78-4103-b5ea-d57841b21f9c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_TemporaryEquityByClassOfStockTable_a968a259-e987-4d2e-96bf-d29e55d6b157" xlink:to="loc_us-gaap_StatementClassOfStockAxis_9ab2679b-2d78-4103-b5ea-d57841b21f9c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_9ab2679b-2d78-4103-b5ea-d57841b21f9c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_9ab2679b-2d78-4103-b5ea-d57841b21f9c" xlink:to="loc_us-gaap_ClassOfStockDomain_9ab2679b-2d78-4103-b5ea-d57841b21f9c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_dc8395d3-99e9-43bd-a9bf-e205256c4ddd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_9ab2679b-2d78-4103-b5ea-d57841b21f9c" xlink:to="loc_us-gaap_ClassOfStockDomain_dc8395d3-99e9-43bd-a9bf-e205256c4ddd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesA1RedeemableConvertiblePreferredStockMember_ef0334e8-d629-4791-9fe1-4275e0ef5f7e" xlink:href="blln-20250930.xsd#blln_SeriesA1RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_dc8395d3-99e9-43bd-a9bf-e205256c4ddd" xlink:to="loc_blln_SeriesA1RedeemableConvertiblePreferredStockMember_ef0334e8-d629-4791-9fe1-4275e0ef5f7e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesA2RedeemableConvertiblePreferredStockMember_246066b5-d7e9-498a-b790-c3db5ccca006" xlink:href="blln-20250930.xsd#blln_SeriesA2RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_dc8395d3-99e9-43bd-a9bf-e205256c4ddd" xlink:to="loc_blln_SeriesA2RedeemableConvertiblePreferredStockMember_246066b5-d7e9-498a-b790-c3db5ccca006" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesA3RedeemableConvertiblePreferredStockMember_7f90e90d-dbca-4a11-bf1e-bf4c46bb1066" xlink:href="blln-20250930.xsd#blln_SeriesA3RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_dc8395d3-99e9-43bd-a9bf-e205256c4ddd" xlink:to="loc_blln_SeriesA3RedeemableConvertiblePreferredStockMember_7f90e90d-dbca-4a11-bf1e-bf4c46bb1066" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesA4RedeemableConvertiblePreferredStockMember_6ecc3304-ac18-4ff1-a840-de449838ea03" xlink:href="blln-20250930.xsd#blln_SeriesA4RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_dc8395d3-99e9-43bd-a9bf-e205256c4ddd" xlink:to="loc_blln_SeriesA4RedeemableConvertiblePreferredStockMember_6ecc3304-ac18-4ff1-a840-de449838ea03" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesA5RedeemableConvertiblePreferredStockMember_82f2ba8f-643a-4533-bdc5-a84524ac0645" xlink:href="blln-20250930.xsd#blln_SeriesA5RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_dc8395d3-99e9-43bd-a9bf-e205256c4ddd" xlink:to="loc_blln_SeriesA5RedeemableConvertiblePreferredStockMember_82f2ba8f-643a-4533-bdc5-a84524ac0645" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesA6RedeemableConvertiblePreferredStockMember_8f8457d8-f005-4540-b3fc-950889c6da3d" xlink:href="blln-20250930.xsd#blln_SeriesA6RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_dc8395d3-99e9-43bd-a9bf-e205256c4ddd" xlink:to="loc_blln_SeriesA6RedeemableConvertiblePreferredStockMember_8f8457d8-f005-4540-b3fc-950889c6da3d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesB1RedeemableConvertiblePreferredStockMember_40264035-453d-4967-9a41-71af41cd03fa" xlink:href="blln-20250930.xsd#blln_SeriesB1RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_dc8395d3-99e9-43bd-a9bf-e205256c4ddd" xlink:to="loc_blln_SeriesB1RedeemableConvertiblePreferredStockMember_40264035-453d-4967-9a41-71af41cd03fa" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesB2RedeemableConvertiblePreferredStockMember_b73f4af0-84c3-4bfa-8143-ab22919611c4" xlink:href="blln-20250930.xsd#blln_SeriesB2RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_dc8395d3-99e9-43bd-a9bf-e205256c4ddd" xlink:to="loc_blln_SeriesB2RedeemableConvertiblePreferredStockMember_b73f4af0-84c3-4bfa-8143-ab22919611c4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesCRedeemableConvertiblePreferredStockMember_4a617a9c-26e0-4453-91e0-fb68698d74eb" xlink:href="blln-20250930.xsd#blln_SeriesCRedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_dc8395d3-99e9-43bd-a9bf-e205256c4ddd" xlink:to="loc_blln_SeriesCRedeemableConvertiblePreferredStockMember_4a617a9c-26e0-4453-91e0-fb68698d74eb" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesC1RedeemableConvertiblePreferredStockMember_8344b318-55cf-48de-919f-3a5b7f549197" xlink:href="blln-20250930.xsd#blln_SeriesC1RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_dc8395d3-99e9-43bd-a9bf-e205256c4ddd" xlink:to="loc_blln_SeriesC1RedeemableConvertiblePreferredStockMember_8344b318-55cf-48de-919f-3a5b7f549197" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesDRedeemableConvertiblePreferredStockMember_7b4d8721-8ce2-41c2-8c05-231961cb3b65" xlink:href="blln-20250930.xsd#blln_SeriesDRedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_dc8395d3-99e9-43bd-a9bf-e205256c4ddd" xlink:to="loc_blln_SeriesDRedeemableConvertiblePreferredStockMember_7b4d8721-8ce2-41c2-8c05-231961cb3b65" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesAuthorized_14727c3d-206e-470c-baf1-c62edc2a7946" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquitySharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TemporaryEquityLineItems_5c696c04-91ac-434c-b5aa-d1a3420e0be5" xlink:to="loc_us-gaap_TemporaryEquitySharesAuthorized_14727c3d-206e-470c-baf1-c62edc2a7946" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesIssued_a3db8c70-5079-4db2-9f49-357573aa0411" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquitySharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TemporaryEquityLineItems_5c696c04-91ac-434c-b5aa-d1a3420e0be5" xlink:to="loc_us-gaap_TemporaryEquitySharesIssued_a3db8c70-5079-4db2-9f49-357573aa0411" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_edda606c-5a50-4a79-84f1-9d2c1d6297b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquitySharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TemporaryEquityLineItems_5c696c04-91ac-434c-b5aa-d1a3420e0be5" xlink:to="loc_us-gaap_TemporaryEquitySharesOutstanding_edda606c-5a50-4a79-84f1-9d2c1d6297b7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountIncludingPortionAttributableToNoncontrollingInterests_9d94e10c-0b4f-4c00-917d-a49eed811484" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityCarryingAmountIncludingPortionAttributableToNoncontrollingInterests"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TemporaryEquityLineItems_5c696c04-91ac-434c-b5aa-d1a3420e0be5" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountIncludingPortionAttributableToNoncontrollingInterests_9d94e10c-0b4f-4c00-917d-a49eed811484" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityAggregateAmountOfRedemptionRequirement_4538b8b4-4c4c-49e6-9c0a-e4bbac20884d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TemporaryEquityAggregateAmountOfRedemptionRequirement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TemporaryEquityLineItems_5c696c04-91ac-434c-b5aa-d1a3420e0be5" xlink:to="loc_us-gaap_TemporaryEquityAggregateAmountOfRedemptionRequirement_4538b8b4-4c4c-49e6-9c0a-e4bbac20884d" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/ConvertibleNotesDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#ConvertibleNotesDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/ConvertibleNotesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_7a3f3e9e-3c82-4ba3-a074-6315f4ffa1ba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_7aa323e6-cabb-4f2b-875d-d20f2fabd1d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_7a3f3e9e-3c82-4ba3-a074-6315f4ffa1ba" xlink:to="loc_us-gaap_DebtInstrumentTable_7aa323e6-cabb-4f2b-875d-d20f2fabd1d9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_6699855b-c970-45b2-8855-8dcd742596ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_7aa323e6-cabb-4f2b-875d-d20f2fabd1d9" xlink:to="loc_us-gaap_DebtInstrumentAxis_6699855b-c970-45b2-8855-8dcd742596ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_6699855b-c970-45b2-8855-8dcd742596ee_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_6699855b-c970-45b2-8855-8dcd742596ee" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_6699855b-c970-45b2-8855-8dcd742596ee_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_d6a49087-22f3-4702-bf48-9007762c89a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_6699855b-c970-45b2-8855-8dcd742596ee" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_d6a49087-22f3-4702-bf48-9007762c89a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_UnsecuredConvertiblePromissoryNotesMember_cf633727-f40b-48a4-ad3a-8dc0caab546c" xlink:href="blln-20250930.xsd#blln_UnsecuredConvertiblePromissoryNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_d6a49087-22f3-4702-bf48-9007762c89a8" xlink:to="loc_blln_UnsecuredConvertiblePromissoryNotesMember_cf633727-f40b-48a4-ad3a-8dc0caab546c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_7786fbb3-2142-4cd0-a115-58cefb31cbb5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_7aa323e6-cabb-4f2b-875d-d20f2fabd1d9" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_7786fbb3-2142-4cd0-a115-58cefb31cbb5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_7786fbb3-2142-4cd0-a115-58cefb31cbb5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_7786fbb3-2142-4cd0-a115-58cefb31cbb5" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_7786fbb3-2142-4cd0-a115-58cefb31cbb5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_748b1cbd-aaa6-45a8-96c0-9877fa14351a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_7786fbb3-2142-4cd0-a115-58cefb31cbb5" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_748b1cbd-aaa6-45a8-96c0-9877fa14351a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_614ca327-40ad-4408-8d7f-03e5bc532f2c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_748b1cbd-aaa6-45a8-96c0-9877fa14351a" xlink:to="loc_us-gaap_ConvertibleDebtMember_614ca327-40ad-4408-8d7f-03e5bc532f2c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_41ed8d15-7d7d-4b22-b402-0de972de203a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_7aa323e6-cabb-4f2b-875d-d20f2fabd1d9" xlink:to="loc_us-gaap_StatementClassOfStockAxis_41ed8d15-7d7d-4b22-b402-0de972de203a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_41ed8d15-7d7d-4b22-b402-0de972de203a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_41ed8d15-7d7d-4b22-b402-0de972de203a" xlink:to="loc_us-gaap_ClassOfStockDomain_41ed8d15-7d7d-4b22-b402-0de972de203a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_1bd35d5d-97b7-44ea-9770-2d9a8db7005d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_41ed8d15-7d7d-4b22-b402-0de972de203a" xlink:to="loc_us-gaap_ClassOfStockDomain_1bd35d5d-97b7-44ea-9770-2d9a8db7005d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesC1RedeemableConvertiblePreferredStockMember_63597280-1c9e-4cd4-ae3e-cd01598eeeed" xlink:href="blln-20250930.xsd#blln_SeriesC1RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_1bd35d5d-97b7-44ea-9770-2d9a8db7005d" xlink:to="loc_blln_SeriesC1RedeemableConvertiblePreferredStockMember_63597280-1c9e-4cd4-ae3e-cd01598eeeed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_16179216-43b9-4e82-acc0-b904926b3a4c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_7a3f3e9e-3c82-4ba3-a074-6315f4ffa1ba" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_16179216-43b9-4e82-acc0-b904926b3a4c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_2a723a60-8396-4326-a940-ed6165655302" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_7a3f3e9e-3c82-4ba3-a074-6315f4ffa1ba" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_2a723a60-8396-4326-a940-ed6165655302" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtConversionConvertedInstrumentSharesIssued1_b1bf8d90-c0ae-4c49-a309-1bbe08e4451c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtConversionConvertedInstrumentSharesIssued1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_7a3f3e9e-3c82-4ba3-a074-6315f4ffa1ba" xlink:to="loc_us-gaap_DebtConversionConvertedInstrumentSharesIssued1_b1bf8d90-c0ae-4c49-a309-1bbe08e4451c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_8f288c46-5445-4544-a773-c8f5daf84bc1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleConversionPrice1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_7a3f3e9e-3c82-4ba3-a074-6315f4ffa1ba" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_8f288c46-5445-4544-a773-c8f5daf84bc1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_FairValueAdjustmentOfConvertibleNotes_a9249e2a-cec9-4ba8-a1ba-83150d72d5ea" xlink:href="blln-20250930.xsd#blln_FairValueAdjustmentOfConvertibleNotes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_7a3f3e9e-3c82-4ba3-a074-6315f4ffa1ba" xlink:to="loc_blln_FairValueAdjustmentOfConvertibleNotes_a9249e2a-cec9-4ba8-a1ba-83150d72d5ea" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtFairValueDisclosures_23dc0968-457f-4d31-a9d9-09451ba3549e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtFairValueDisclosures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_7a3f3e9e-3c82-4ba3-a074-6315f4ffa1ba" xlink:to="loc_us-gaap_ConvertibleDebtFairValueDisclosures_23dc0968-457f-4d31-a9d9-09451ba3549e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_861c0972-e826-423f-ad5a-1160f646fd70" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_7a3f3e9e-3c82-4ba3-a074-6315f4ffa1ba" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_861c0972-e826-423f-ad5a-1160f646fd70" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityFairValueDisclosure_78020ad7-9896-4f08-abb0-6c28336d265f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_7a3f3e9e-3c82-4ba3-a074-6315f4ffa1ba" xlink:to="loc_us-gaap_EquityFairValueDisclosure_78020ad7-9896-4f08-abb0-6c28336d265f" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/LongtermDebtNarrativeDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#LongtermDebtNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/LongtermDebtNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_8b51dc29-c38a-4e1d-aad6-f2cf02e5732a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_us-gaap_DebtInstrumentTable_8b51dc29-c38a-4e1d-aad6-f2cf02e5732a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_d3a6c0b4-6509-4314-8a04-05189971cd02" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8b51dc29-c38a-4e1d-aad6-f2cf02e5732a" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_d3a6c0b4-6509-4314-8a04-05189971cd02" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_d3a6c0b4-6509-4314-8a04-05189971cd02_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_d3a6c0b4-6509-4314-8a04-05189971cd02" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_d3a6c0b4-6509-4314-8a04-05189971cd02_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_745643e6-2280-491c-8546-2ab24d6fe44b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_d3a6c0b4-6509-4314-8a04-05189971cd02" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_745643e6-2280-491c-8546-2ab24d6fe44b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayableOtherPayablesMember_40530bc5-2186-4164-8cd8-3e2c5e8feeec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesPayableOtherPayablesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_745643e6-2280-491c-8546-2ab24d6fe44b" xlink:to="loc_us-gaap_NotesPayableOtherPayablesMember_40530bc5-2186-4164-8cd8-3e2c5e8feeec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_397b6d22-f611-4a6e-81af-e537f88ff552" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8b51dc29-c38a-4e1d-aad6-f2cf02e5732a" xlink:to="loc_us-gaap_DebtInstrumentAxis_397b6d22-f611-4a6e-81af-e537f88ff552" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_397b6d22-f611-4a6e-81af-e537f88ff552_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_397b6d22-f611-4a6e-81af-e537f88ff552" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_397b6d22-f611-4a6e-81af-e537f88ff552_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_b7695f32-c2c3-4ae8-8eb5-ad67708f3d36" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_397b6d22-f611-4a6e-81af-e537f88ff552" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_b7695f32-c2c3-4ae8-8eb5-ad67708f3d36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_A2024NotesMember_3ddbbd42-0eb4-4dc9-b330-d523df38e083" xlink:href="blln-20250930.xsd#blln_A2024NotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_b7695f32-c2c3-4ae8-8eb5-ad67708f3d36" xlink:to="loc_blln_A2024NotesMember_3ddbbd42-0eb4-4dc9-b330-d523df38e083" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtCovenantTrancheAxis_42365b55-4df4-4a04-888c-5e4103394833" xlink:href="blln-20250930.xsd#blln_DebtCovenantTrancheAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8b51dc29-c38a-4e1d-aad6-f2cf02e5732a" xlink:to="loc_blln_DebtCovenantTrancheAxis_42365b55-4df4-4a04-888c-5e4103394833" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtCovenantTrancheDomain_42365b55-4df4-4a04-888c-5e4103394833_default" xlink:href="blln-20250930.xsd#blln_DebtCovenantTrancheDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_blln_DebtCovenantTrancheAxis_42365b55-4df4-4a04-888c-5e4103394833" xlink:to="loc_blln_DebtCovenantTrancheDomain_42365b55-4df4-4a04-888c-5e4103394833_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtCovenantTrancheDomain_8dfc7ff8-d682-43ce-88c1-f889139444a3" xlink:href="blln-20250930.xsd#blln_DebtCovenantTrancheDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_blln_DebtCovenantTrancheAxis_42365b55-4df4-4a04-888c-5e4103394833" xlink:to="loc_blln_DebtCovenantTrancheDomain_8dfc7ff8-d682-43ce-88c1-f889139444a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtCovenantTrancheOneMember_1ea5050e-86ac-4515-8fc2-d46e35bc4a1a" xlink:href="blln-20250930.xsd#blln_DebtCovenantTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_blln_DebtCovenantTrancheDomain_8dfc7ff8-d682-43ce-88c1-f889139444a3" xlink:to="loc_blln_DebtCovenantTrancheOneMember_1ea5050e-86ac-4515-8fc2-d46e35bc4a1a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtCovenantTrancheTwoMember_e309bbad-fcc9-47e7-bee2-b0bfaab75b41" xlink:href="blln-20250930.xsd#blln_DebtCovenantTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_blln_DebtCovenantTrancheDomain_8dfc7ff8-d682-43ce-88c1-f889139444a3" xlink:to="loc_blln_DebtCovenantTrancheTwoMember_e309bbad-fcc9-47e7-bee2-b0bfaab75b41" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtCovenantTrancheThreeMember_b3f25822-0aaa-425f-b63c-c562cabbfc52" xlink:href="blln-20250930.xsd#blln_DebtCovenantTrancheThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_blln_DebtCovenantTrancheDomain_8dfc7ff8-d682-43ce-88c1-f889139444a3" xlink:to="loc_blln_DebtCovenantTrancheThreeMember_b3f25822-0aaa-425f-b63c-c562cabbfc52" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtCovenantTrancheFourMember_4522e19a-cc34-4eb3-9f3d-1a045c91f65f" xlink:href="blln-20250930.xsd#blln_DebtCovenantTrancheFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_blln_DebtCovenantTrancheDomain_8dfc7ff8-d682-43ce-88c1-f889139444a3" xlink:to="loc_blln_DebtCovenantTrancheFourMember_4522e19a-cc34-4eb3-9f3d-1a045c91f65f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_6c4fe37b-9dad-48e2-aede-ed20251d81c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8b51dc29-c38a-4e1d-aad6-f2cf02e5732a" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_6c4fe37b-9dad-48e2-aede-ed20251d81c6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_6c4fe37b-9dad-48e2-aede-ed20251d81c6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_6c4fe37b-9dad-48e2-aede-ed20251d81c6" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_6c4fe37b-9dad-48e2-aede-ed20251d81c6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_3683e7af-8902-4124-b42e-b2e5499612c9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_6c4fe37b-9dad-48e2-aede-ed20251d81c6" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_3683e7af-8902-4124-b42e-b2e5499612c9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodOneMember_89aebc52-1b7b-4509-83d0-a2549edefabe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_3683e7af-8902-4124-b42e-b2e5499612c9" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodOneMember_89aebc52-1b7b-4509-83d0-a2549edefabe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodTwoMember_a0fa2c89-ade7-4a95-9bb9-0e718fcb56bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_3683e7af-8902-4124-b42e-b2e5499612c9" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodTwoMember_a0fa2c89-ade7-4a95-9bb9-0e718fcb56bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodThreeMember_f3cc17f1-284c-485f-9adc-3a2d7855e68c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_3683e7af-8902-4124-b42e-b2e5499612c9" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodThreeMember_f3cc17f1-284c-485f-9adc-3a2d7855e68c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodFourMember_31a1c48e-88d8-4096-ada7-e2706fd440ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_3683e7af-8902-4124-b42e-b2e5499612c9" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodFourMember_31a1c48e-88d8-4096-ada7-e2706fd440ad" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodFiveMember_10075474-618b-4cad-979b-50e420ab4893" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPeriodFiveMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_3683e7af-8902-4124-b42e-b2e5499612c9" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodFiveMember_10075474-618b-4cad-979b-50e420ab4893" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtInstrumentRedemptionPeriodSixMember_2ef3a480-56a9-4397-87ef-7276445a296e" xlink:href="blln-20250930.xsd#blln_DebtInstrumentRedemptionPeriodSixMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_3683e7af-8902-4124-b42e-b2e5499612c9" xlink:to="loc_blln_DebtInstrumentRedemptionPeriodSixMember_2ef3a480-56a9-4397-87ef-7276445a296e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_ff894baa-57b5-4196-b6ea-78304a6425d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8b51dc29-c38a-4e1d-aad6-f2cf02e5732a" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_ff894baa-57b5-4196-b6ea-78304a6425d9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_ff894baa-57b5-4196-b6ea-78304a6425d9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_ff894baa-57b5-4196-b6ea-78304a6425d9" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_ff894baa-57b5-4196-b6ea-78304a6425d9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_4150650c-05db-46ee-aeea-d08ef610cc35" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_ff894baa-57b5-4196-b6ea-78304a6425d9" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_4150650c-05db-46ee-aeea-d08ef610cc35" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_RedeemableConvertiblePreferredStockWarrantsMember_4c320461-1439-4368-af22-0228cc50c1b3" xlink:href="blln-20250930.xsd#blln_RedeemableConvertiblePreferredStockWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_4150650c-05db-46ee-aeea-d08ef610cc35" xlink:to="loc_blln_RedeemableConvertiblePreferredStockWarrantsMember_4c320461-1439-4368-af22-0228cc50c1b3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_A2021LSACommonStockWarrantsMember_96f4fca5-4d72-4d11-8f5a-fa5c3fcffede" xlink:href="blln-20250930.xsd#blln_A2021LSACommonStockWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_4150650c-05db-46ee-aeea-d08ef610cc35" xlink:to="loc_blln_A2021LSACommonStockWarrantsMember_96f4fca5-4d72-4d11-8f5a-fa5c3fcffede" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_A2022LSACommonStockWarrantsMember_d6820ba2-3360-4abf-b7c0-72177ec89339" xlink:href="blln-20250930.xsd#blln_A2022LSACommonStockWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_4150650c-05db-46ee-aeea-d08ef610cc35" xlink:to="loc_blln_A2022LSACommonStockWarrantsMember_d6820ba2-3360-4abf-b7c0-72177ec89339" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_4abe0a2b-f78a-4984-a787-43617c01e718" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8b51dc29-c38a-4e1d-aad6-f2cf02e5732a" xlink:to="loc_srt_CounterpartyNameAxis_4abe0a2b-f78a-4984-a787-43617c01e718" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_4abe0a2b-f78a-4984-a787-43617c01e718_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_4abe0a2b-f78a-4984-a787-43617c01e718" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_4abe0a2b-f78a-4984-a787-43617c01e718_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_7b33ac0a-6f81-47d6-9d0b-85f0dc094cfd" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_4abe0a2b-f78a-4984-a787-43617c01e718" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_7b33ac0a-6f81-47d6-9d0b-85f0dc094cfd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_OberlandCapitalMember_d846a8e4-5aba-4bd8-9f32-1ab1c08b7ea4" xlink:href="blln-20250930.xsd#blln_OberlandCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_7b33ac0a-6f81-47d6-9d0b-85f0dc094cfd" xlink:to="loc_blln_OberlandCapitalMember_d846a8e4-5aba-4bd8-9f32-1ab1c08b7ea4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_e04e7b1d-9f56-48f3-a4f8-640c558e41b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_e04e7b1d-9f56-48f3-a4f8-640c558e41b9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_576d7371-1a63-4bf9-946b-a0e069592d32" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_576d7371-1a63-4bf9-946b-a0e069592d32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromNotesPayable_b5a33ce8-5d16-451d-b96b-d1f747e61349" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromNotesPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_us-gaap_ProceedsFromNotesPayable_b5a33ce8-5d16-451d-b96b-d1f747e61349" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtInstrumentCovenantTrailingSixMonthWorldwideNetRevenueMinimum_3fcc4e79-5b98-457e-a0a4-4db32e026e75" xlink:href="blln-20250930.xsd#blln_DebtInstrumentCovenantTrailingSixMonthWorldwideNetRevenueMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_DebtInstrumentCovenantTrailingSixMonthWorldwideNetRevenueMinimum_3fcc4e79-5b98-457e-a0a4-4db32e026e75" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtInstrumentCovenantTrailingSixMonthGrossMarginMinimum_f41c3e1d-f716-4a29-8be3-5169a61de9b6" xlink:href="blln-20250930.xsd#blln_DebtInstrumentCovenantTrailingSixMonthGrossMarginMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_DebtInstrumentCovenantTrailingSixMonthGrossMarginMinimum_f41c3e1d-f716-4a29-8be3-5169a61de9b6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_02cdeee2-a0cb-495c-ad49-cbe6c1bcea59" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentRedemptionPricePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_02cdeee2-a0cb-495c-ad49-cbe6c1bcea59" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtInstrumentRedemptionPeriod_ff029cc6-4353-469d-9b6d-d1b6ad85e90d" xlink:href="blln-20250930.xsd#blln_DebtInstrumentRedemptionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_DebtInstrumentRedemptionPeriod_ff029cc6-4353-469d-9b6d-d1b6ad85e90d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtInstrumentRedemptionInternalRateOfReturnGenerated_58ee4763-df70-4e4e-ab50-3d4bc6856980" xlink:href="blln-20250930.xsd#blln_DebtInstrumentRedemptionInternalRateOfReturnGenerated"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_DebtInstrumentRedemptionInternalRateOfReturnGenerated_58ee4763-df70-4e4e-ab50-3d4bc6856980" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtInstrumentRevenueParticipationAgreementPercentOfQuarterlyNetRevenue_1c3592f0-aa1a-45d6-ab9c-41c06a27718e" xlink:href="blln-20250930.xsd#blln_DebtInstrumentRevenueParticipationAgreementPercentOfQuarterlyNetRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_DebtInstrumentRevenueParticipationAgreementPercentOfQuarterlyNetRevenue_1c3592f0-aa1a-45d6-ab9c-41c06a27718e" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtInstrumentRevenueParticipationAgreementMaximumAnnualNetRevenue_85c21403-56d8-4777-a751-8a48ec177eeb" xlink:href="blln-20250930.xsd#blln_DebtInstrumentRevenueParticipationAgreementMaximumAnnualNetRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_DebtInstrumentRevenueParticipationAgreementMaximumAnnualNetRevenue_85c21403-56d8-4777-a751-8a48ec177eeb" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtInstrumentRevenueParticipationAgreementPrincipalAmountOfNotes_02623c3c-f155-4d52-b72c-6dd14e80e3a8" xlink:href="blln-20250930.xsd#blln_DebtInstrumentRevenueParticipationAgreementPrincipalAmountOfNotes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_DebtInstrumentRevenueParticipationAgreementPrincipalAmountOfNotes_02623c3c-f155-4d52-b72c-6dd14e80e3a8" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_PaymentsForRevenueParticipationAgreement_8e71e93b-e49c-450b-989a-0833214605f8" xlink:href="blln-20250930.xsd#blln_PaymentsForRevenueParticipationAgreement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_PaymentsForRevenueParticipationAgreement_8e71e93b-e49c-450b-989a-0833214605f8" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_5a0632e3-0bd0-425a-938f-62ad3455ddde" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_us-gaap_InterestExpenseDebt_5a0632e3-0bd0-425a-938f-62ad3455ddde" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_a690ac13-d6ae-4634-aa47-2f5d298949cd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_us-gaap_LongTermDebt_a690ac13-d6ae-4634-aa47-2f5d298949cd" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive_20a84b41-06af-4454-8466-3101c87dd060" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive_20a84b41-06af-4454-8466-3101c87dd060" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_aa48be16-5084-4f78-b417-b0444d8a0e81" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_aa48be16-5084-4f78-b417-b0444d8a0e81" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_c5a30eb6-54e2-4146-a342-d0fe0375a2ef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_c5a30eb6-54e2-4146-a342-d0fe0375a2ef" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_LoansPayableCapitalAdvancesNumberOfTranches_dbb64327-b4b1-41cd-b06e-ccfcdea02bb7" xlink:href="blln-20250930.xsd#blln_LoansPayableCapitalAdvancesNumberOfTranches"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_LoansPayableCapitalAdvancesNumberOfTranches_dbb64327-b4b1-41cd-b06e-ccfcdea02bb7" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LoansPayableToBank_5e969ea0-3c18-4ca9-81c0-1e69a7b98711" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LoansPayableToBank"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_us-gaap_LoansPayableToBank_5e969ea0-3c18-4ca9-81c0-1e69a7b98711" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForLoans_b609e17b-e1cd-408b-9683-4d081c8b1467" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForLoans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_us-gaap_PaymentsForLoans_b609e17b-e1cd-408b-9683-4d081c8b1467" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_PaymentsOfLoanTerminationFees_29a68557-43d8-4445-bd1c-338c96cedc8c" xlink:href="blln-20250930.xsd#blln_PaymentsOfLoanTerminationFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_PaymentsOfLoanTerminationFees_29a68557-43d8-4445-bd1c-338c96cedc8c" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_a2bf7593-a2b9-433a-89e4-e6c841722d77" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_a2bf7593-a2b9-433a-89e4-e6c841722d77" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_LossOnExtinguishmentOfDebtIncrementalExpense_e26a40dd-d33c-490d-8579-e6eb44ecdb42" xlink:href="blln-20250930.xsd#blln_LossOnExtinguishmentOfDebtIncrementalExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_LossOnExtinguishmentOfDebtIncrementalExpense_e26a40dd-d33c-490d-8579-e6eb44ecdb42" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_LossOnExtinguishmentOfDebtUnamortizedDebtIssuanceCosts_ba3e95ee-e734-4d39-b4e4-a7f661f9e53d" xlink:href="blln-20250930.xsd#blln_LossOnExtinguishmentOfDebtUnamortizedDebtIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_LossOnExtinguishmentOfDebtUnamortizedDebtIssuanceCosts_ba3e95ee-e734-4d39-b4e4-a7f661f9e53d" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_LossOnExtinguishmentOfDebtPrepaymentFee_afdf169a-2b05-41fe-96aa-8f91787f14e4" xlink:href="blln-20250930.xsd#blln_LossOnExtinguishmentOfDebtPrepaymentFee"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_LossOnExtinguishmentOfDebtPrepaymentFee_afdf169a-2b05-41fe-96aa-8f91787f14e4" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_ClassOfWarrantOrRightRightToReturnWarrantsToCompanyCashValue_b702d043-9fb4-48d5-b823-83eb354e37a5" xlink:href="blln-20250930.xsd#blln_ClassOfWarrantOrRightRightToReturnWarrantsToCompanyCashValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_ClassOfWarrantOrRightRightToReturnWarrantsToCompanyCashValue_b702d043-9fb4-48d5-b823-83eb354e37a5" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_ClassOfWarrantOrRightSharesWarrantsIssuedDuringPeriod_d74e5c0d-934f-4446-aec7-17a53b4f9a4c" xlink:href="blln-20250930.xsd#blln_ClassOfWarrantOrRightSharesWarrantsIssuedDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_blln_ClassOfWarrantOrRightSharesWarrantsIssuedDuringPeriod_d74e5c0d-934f-4446-aec7-17a53b4f9a4c" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightUnissued_4cde0c2e-9f73-42b8-9efc-360efb912a96" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightUnissued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_6b62e7e2-898c-4bd8-a950-4fb852fba01a" xlink:to="loc_us-gaap_ClassOfWarrantOrRightUnissued_4cde0c2e-9f73-42b8-9efc-360efb912a96" xlink:type="arc" order="27"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/LongtermDebtScheduleofNotesPayableCovenantsDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#LongtermDebtScheduleofNotesPayableCovenantsDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/LongtermDebtScheduleofNotesPayableCovenantsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_b4feeb92-6355-43e5-8cdd-160146370920" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_8cec7dba-0f38-4995-9262-2c78a02276c8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4feeb92-6355-43e5-8cdd-160146370920" xlink:to="loc_us-gaap_DebtInstrumentTable_8cec7dba-0f38-4995-9262-2c78a02276c8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_c8d69e80-fcd3-451a-8db7-2c7bce32a992" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8cec7dba-0f38-4995-9262-2c78a02276c8" xlink:to="loc_us-gaap_DebtInstrumentAxis_c8d69e80-fcd3-451a-8db7-2c7bce32a992" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_c8d69e80-fcd3-451a-8db7-2c7bce32a992_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_c8d69e80-fcd3-451a-8db7-2c7bce32a992" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_c8d69e80-fcd3-451a-8db7-2c7bce32a992_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_b6c5bbdb-ee5a-43f8-9c39-c39089d21e8a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_c8d69e80-fcd3-451a-8db7-2c7bce32a992" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_b6c5bbdb-ee5a-43f8-9c39-c39089d21e8a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_A2024NotesMember_8c09a8ce-3ee8-44e6-9f77-a500927c6673" xlink:href="blln-20250930.xsd#blln_A2024NotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_b6c5bbdb-ee5a-43f8-9c39-c39089d21e8a" xlink:to="loc_blln_A2024NotesMember_8c09a8ce-3ee8-44e6-9f77-a500927c6673" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_9aab4736-a292-486f-a11d-6c01551b4b09" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8cec7dba-0f38-4995-9262-2c78a02276c8" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_9aab4736-a292-486f-a11d-6c01551b4b09" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_9aab4736-a292-486f-a11d-6c01551b4b09_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_9aab4736-a292-486f-a11d-6c01551b4b09" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_9aab4736-a292-486f-a11d-6c01551b4b09_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_80318562-fd31-41ca-aa2b-2b5683237ba8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_9aab4736-a292-486f-a11d-6c01551b4b09" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_80318562-fd31-41ca-aa2b-2b5683237ba8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayableOtherPayablesMember_8b33fc38-23a7-4102-ba11-b1dbf7caaff2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NotesPayableOtherPayablesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_80318562-fd31-41ca-aa2b-2b5683237ba8" xlink:to="loc_us-gaap_NotesPayableOtherPayablesMember_8b33fc38-23a7-4102-ba11-b1dbf7caaff2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtInstrumentCovenantScheduleAxis_665bd2b1-7403-484f-a075-1a0575c10018" xlink:href="blln-20250930.xsd#blln_DebtInstrumentCovenantScheduleAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8cec7dba-0f38-4995-9262-2c78a02276c8" xlink:to="loc_blln_DebtInstrumentCovenantScheduleAxis_665bd2b1-7403-484f-a075-1a0575c10018" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtInstrumentCovenantScheduleDomain_665bd2b1-7403-484f-a075-1a0575c10018_default" xlink:href="blln-20250930.xsd#blln_DebtInstrumentCovenantScheduleDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_blln_DebtInstrumentCovenantScheduleAxis_665bd2b1-7403-484f-a075-1a0575c10018" xlink:to="loc_blln_DebtInstrumentCovenantScheduleDomain_665bd2b1-7403-484f-a075-1a0575c10018_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtInstrumentCovenantScheduleDomain_ef68d196-cbef-4f38-9d0d-aecf83da3e4c" xlink:href="blln-20250930.xsd#blln_DebtInstrumentCovenantScheduleDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_blln_DebtInstrumentCovenantScheduleAxis_665bd2b1-7403-484f-a075-1a0575c10018" xlink:to="loc_blln_DebtInstrumentCovenantScheduleDomain_ef68d196-cbef-4f38-9d0d-aecf83da3e4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_Q12025Member_382b3282-b9b3-44fd-821f-97c75702f8e4" xlink:href="blln-20250930.xsd#blln_Q12025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_blln_DebtInstrumentCovenantScheduleDomain_ef68d196-cbef-4f38-9d0d-aecf83da3e4c" xlink:to="loc_blln_Q12025Member_382b3282-b9b3-44fd-821f-97c75702f8e4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_Q22025Member_7a1e4436-190d-431f-a872-2ad7f8670976" xlink:href="blln-20250930.xsd#blln_Q22025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_blln_DebtInstrumentCovenantScheduleDomain_ef68d196-cbef-4f38-9d0d-aecf83da3e4c" xlink:to="loc_blln_Q22025Member_7a1e4436-190d-431f-a872-2ad7f8670976" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_Q32025Member_4a870ac8-4678-4617-af6e-64ebadb08d58" xlink:href="blln-20250930.xsd#blln_Q32025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_blln_DebtInstrumentCovenantScheduleDomain_ef68d196-cbef-4f38-9d0d-aecf83da3e4c" xlink:to="loc_blln_Q32025Member_4a870ac8-4678-4617-af6e-64ebadb08d58" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_Q42025Member_ef1d739e-13dc-4094-ac75-f36efa08b0b4" xlink:href="blln-20250930.xsd#blln_Q42025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_blln_DebtInstrumentCovenantScheduleDomain_ef68d196-cbef-4f38-9d0d-aecf83da3e4c" xlink:to="loc_blln_Q42025Member_ef1d739e-13dc-4094-ac75-f36efa08b0b4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_Q12026Member_e60107cf-190f-4602-8608-bdd3e88c5e74" xlink:href="blln-20250930.xsd#blln_Q12026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_blln_DebtInstrumentCovenantScheduleDomain_ef68d196-cbef-4f38-9d0d-aecf83da3e4c" xlink:to="loc_blln_Q12026Member_e60107cf-190f-4602-8608-bdd3e88c5e74" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_Q22026Member_6051d010-437d-4f2a-ac08-a639dcdcba2d" xlink:href="blln-20250930.xsd#blln_Q22026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_blln_DebtInstrumentCovenantScheduleDomain_ef68d196-cbef-4f38-9d0d-aecf83da3e4c" xlink:to="loc_blln_Q22026Member_6051d010-437d-4f2a-ac08-a639dcdcba2d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_Q32026Member_19d3a3b5-0b9a-44b6-a51b-49f27a6e0040" xlink:href="blln-20250930.xsd#blln_Q32026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_blln_DebtInstrumentCovenantScheduleDomain_ef68d196-cbef-4f38-9d0d-aecf83da3e4c" xlink:to="loc_blln_Q32026Member_19d3a3b5-0b9a-44b6-a51b-49f27a6e0040" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_Q42026Member_2f8a2d99-8d7e-4f00-95a0-50c8ab31fc15" xlink:href="blln-20250930.xsd#blln_Q42026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_blln_DebtInstrumentCovenantScheduleDomain_ef68d196-cbef-4f38-9d0d-aecf83da3e4c" xlink:to="loc_blln_Q42026Member_2f8a2d99-8d7e-4f00-95a0-50c8ab31fc15" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_ThereafterMember_d0b1580e-6686-4eaa-accb-e8e15868cc28" xlink:href="blln-20250930.xsd#blln_ThereafterMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_blln_DebtInstrumentCovenantScheduleDomain_ef68d196-cbef-4f38-9d0d-aecf83da3e4c" xlink:to="loc_blln_ThereafterMember_d0b1580e-6686-4eaa-accb-e8e15868cc28" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_DebtInstrumentCovenantTrailingSixMonthWorldwideNetRevenueMinimum_aa15fe28-06d4-4061-9e84-8f51ca611f22" xlink:href="blln-20250930.xsd#blln_DebtInstrumentCovenantTrailingSixMonthWorldwideNetRevenueMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_b4feeb92-6355-43e5-8cdd-160146370920" xlink:to="loc_blln_DebtInstrumentCovenantTrailingSixMonthWorldwideNetRevenueMinimum_aa15fe28-06d4-4061-9e84-8f51ca611f22" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/CommonStockNarrativeDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#CommonStockNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/CommonStockNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_e3014d6e-cad9-4f09-8abe-8e53770eda1d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_e3014d6e-cad9-4f09-8abe-8e53770eda1d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_0512be74-6517-46da-a6a0-b623b08baba4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_e3014d6e-cad9-4f09-8abe-8e53770eda1d" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_0512be74-6517-46da-a6a0-b623b08baba4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_0512be74-6517-46da-a6a0-b623b08baba4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_0512be74-6517-46da-a6a0-b623b08baba4" xlink:to="loc_us-gaap_RelatedPartyDomain_0512be74-6517-46da-a6a0-b623b08baba4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_ca298fb1-c5db-4724-87e4-0252bd277fc4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_0512be74-6517-46da-a6a0-b623b08baba4" xlink:to="loc_us-gaap_RelatedPartyDomain_ca298fb1-c5db-4724-87e4-0252bd277fc4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_ChiefExecutiveOfficerAndCoFounderMember_3a5fa452-d37c-4d91-b002-f9ec4cc86341" xlink:href="blln-20250930.xsd#blln_ChiefExecutiveOfficerAndCoFounderMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_ca298fb1-c5db-4724-87e4-0252bd277fc4" xlink:to="loc_blln_ChiefExecutiveOfficerAndCoFounderMember_3a5fa452-d37c-4d91-b002-f9ec4cc86341" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_ChiefTechnologyOfficerAndCoFounderMember_3e3cd4d4-e1eb-447b-8d02-41cdbeba6f78" xlink:href="blln-20250930.xsd#blln_ChiefTechnologyOfficerAndCoFounderMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_ca298fb1-c5db-4724-87e4-0252bd277fc4" xlink:to="loc_blln_ChiefTechnologyOfficerAndCoFounderMember_3e3cd4d4-e1eb-447b-8d02-41cdbeba6f78" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_0d4020b2-701e-481f-bbc4-ea99f488821d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_ca298fb1-c5db-4724-87e4-0252bd277fc4" xlink:to="loc_us-gaap_RelatedPartyMember_0d4020b2-701e-481f-bbc4-ea99f488821d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_610a88d1-f677-4d09-a32f-2de5f8af1201" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_e3014d6e-cad9-4f09-8abe-8e53770eda1d" xlink:to="loc_us-gaap_PlanNameAxis_610a88d1-f677-4d09-a32f-2de5f8af1201" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_610a88d1-f677-4d09-a32f-2de5f8af1201_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_610a88d1-f677-4d09-a32f-2de5f8af1201" xlink:to="loc_us-gaap_PlanNameDomain_610a88d1-f677-4d09-a32f-2de5f8af1201_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_a189be9a-1509-4974-9df3-fedb493ea700" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_610a88d1-f677-4d09-a32f-2de5f8af1201" xlink:to="loc_us-gaap_PlanNameDomain_a189be9a-1509-4974-9df3-fedb493ea700" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_The2018PlanMember_1defc947-9e12-4ea1-8b4a-e927bfc724d4" xlink:href="blln-20250930.xsd#blln_The2018PlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_a189be9a-1509-4974-9df3-fedb493ea700" xlink:to="loc_blln_The2018PlanMember_1defc947-9e12-4ea1-8b4a-e927bfc724d4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_801aa400-8db7-4941-bb19-b50a17adb9fc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_e3014d6e-cad9-4f09-8abe-8e53770eda1d" xlink:to="loc_us-gaap_VestingAxis_801aa400-8db7-4941-bb19-b50a17adb9fc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_801aa400-8db7-4941-bb19-b50a17adb9fc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_801aa400-8db7-4941-bb19-b50a17adb9fc" xlink:to="loc_us-gaap_VestingDomain_801aa400-8db7-4941-bb19-b50a17adb9fc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_5dfe3719-f4ef-4b1b-bba2-55aa0248d8c5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_801aa400-8db7-4941-bb19-b50a17adb9fc" xlink:to="loc_us-gaap_VestingDomain_5dfe3719-f4ef-4b1b-bba2-55aa0248d8c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_7b15880c-ea1f-4d69-80c5-e5628095a7e5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_5dfe3719-f4ef-4b1b-bba2-55aa0248d8c5" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_7b15880c-ea1f-4d69-80c5-e5628095a7e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_99562701-b341-4865-b2bc-29667375cee9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_5dfe3719-f4ef-4b1b-bba2-55aa0248d8c5" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_99562701-b341-4865-b2bc-29667375cee9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_3794dd3f-8f8e-4dac-b60e-eaf9bcdaadc5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_e3014d6e-cad9-4f09-8abe-8e53770eda1d" xlink:to="loc_us-gaap_AwardTypeAxis_3794dd3f-8f8e-4dac-b60e-eaf9bcdaadc5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3794dd3f-8f8e-4dac-b60e-eaf9bcdaadc5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_3794dd3f-8f8e-4dac-b60e-eaf9bcdaadc5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3794dd3f-8f8e-4dac-b60e-eaf9bcdaadc5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_454fac0f-2643-4d60-ad5c-7d4cc9d8fada" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_3794dd3f-8f8e-4dac-b60e-eaf9bcdaadc5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_454fac0f-2643-4d60-ad5c-7d4cc9d8fada" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_17cdeb3b-87d0-4474-a4e2-8c187f4bff9b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_454fac0f-2643-4d60-ad5c-7d4cc9d8fada" xlink:to="loc_us-gaap_EmployeeStockOptionMember_17cdeb3b-87d0-4474-a4e2-8c187f4bff9b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_05dd3e8a-1c7d-4fc4-bf57-6806fa3050d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_05dd3e8a-1c7d-4fc4-bf57-6806fa3050d6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_5f8723f0-7428-409c-a19e-0780de76744e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_5f8723f0-7428-409c-a19e-0780de76744e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_8d23f8dd-71d0-41fe-97d6-43626965ff96" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_8d23f8dd-71d0-41fe-97d6-43626965ff96" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_CommonStockNumberOfVotesPerShare_5d10774b-5c16-4603-8203-5aa295d35a11" xlink:href="blln-20250930.xsd#blln_CommonStockNumberOfVotesPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_blln_CommonStockNumberOfVotesPerShare_5d10774b-5c16-4603-8203-5aa295d35a11" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_68495657-ddce-4a32-9473-5cadf7fb5db4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_68495657-ddce-4a32-9473-5cadf7fb5db4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_3d50e741-231a-42da-95d5-10bd61f92da6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_3d50e741-231a-42da-95d5-10bd61f92da6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_634033d2-575c-4362-8853-a6b214250410" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_634033d2-575c-4362-8853-a6b214250410" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_4e39ad98-434b-4934-afec-af5995012aea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_4e39ad98-434b-4934-afec-af5995012aea" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_3eeeab6e-8afc-4cc5-835f-4cef47ccdb3b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_3eeeab6e-8afc-4cc5-835f-4cef47ccdb3b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_7598d5fb-5d1e-4613-83a3-a2ee618079c2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_7598d5fb-5d1e-4613-83a3-a2ee618079c2" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_f1327229-02e3-48c0-80e3-1dc4b6478973" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_f1327229-02e3-48c0-80e3-1dc4b6478973" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_65340659-611c-4e22-88cd-0772ed7442be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_65340659-611c-4e22-88cd-0772ed7442be" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_fa5f8f34-91d6-43dc-99ee-3dc5318b4549" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_fa5f8f34-91d6-43dc-99ee-3dc5318b4549" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_8df477b8-1514-42f4-adbf-09e4163c184f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_8df477b8-1514-42f4-adbf-09e4163c184f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_9bf49897-6915-4ad0-8b05-23baa2f3c608" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockAcquiredAverageCostPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_9bf49897-6915-4ad0-8b05-23baa2f3c608" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SaleOfStockLossOnSale_b6ad46ae-141b-4497-b9b8-dcfd2afa073c" xlink:href="blln-20250930.xsd#blln_SaleOfStockLossOnSale"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_blln_SaleOfStockLossOnSale_b6ad46ae-141b-4497-b9b8-dcfd2afa073c" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodShares_f2e43701-fa22-4497-99ee-f50fae3bf742" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodShares_f2e43701-fa22-4497-99ee-f50fae3bf742" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodValue_b80cd78b-cacb-445b-91c8-a03aff54fe1f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodValue_b80cd78b-cacb-445b-91c8-a03aff54fe1f" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_2f91c8d9-c52e-48bc-8428-60846fee3306" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a2190583-3339-4f90-b964-0340293e8091" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_2f91c8d9-c52e-48bc-8428-60846fee3306" xlink:type="arc" order="18"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/CommonStockReservedforFutureIssuanceDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#CommonStockReservedforFutureIssuanceDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/CommonStockReservedforFutureIssuanceDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_4d294199-d2d8-4bb7-bcee-0fbf526efd3b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_2e6fbdc6-6bb5-46a6-83b6-b698fdc0518c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_4d294199-d2d8-4bb7-bcee-0fbf526efd3b" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_2e6fbdc6-6bb5-46a6-83b6-b698fdc0518c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_602ffd67-b5ff-4ed3-a1c8-336042de1926" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_2e6fbdc6-6bb5-46a6-83b6-b698fdc0518c" xlink:to="loc_us-gaap_StatementClassOfStockAxis_602ffd67-b5ff-4ed3-a1c8-336042de1926" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_602ffd67-b5ff-4ed3-a1c8-336042de1926_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_602ffd67-b5ff-4ed3-a1c8-336042de1926" xlink:to="loc_us-gaap_ClassOfStockDomain_602ffd67-b5ff-4ed3-a1c8-336042de1926_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_602ffd67-b5ff-4ed3-a1c8-336042de1926" xlink:to="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesA1RedeemableConvertiblePreferredStockMember_22cfd626-86c3-45a3-b98e-b53758055542" xlink:href="blln-20250930.xsd#blln_SeriesA1RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_SeriesA1RedeemableConvertiblePreferredStockMember_22cfd626-86c3-45a3-b98e-b53758055542" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesA2RedeemableConvertiblePreferredStockMember_47166dcd-1d37-48ad-babc-f043786298b1" xlink:href="blln-20250930.xsd#blln_SeriesA2RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_SeriesA2RedeemableConvertiblePreferredStockMember_47166dcd-1d37-48ad-babc-f043786298b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesA3RedeemableConvertiblePreferredStockMember_3cf52334-bf1b-4ec7-9d47-d136396f066a" xlink:href="blln-20250930.xsd#blln_SeriesA3RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_SeriesA3RedeemableConvertiblePreferredStockMember_3cf52334-bf1b-4ec7-9d47-d136396f066a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesA4RedeemableConvertiblePreferredStockMember_200bdb60-07e7-4fb6-83ef-2d7a792f46fb" xlink:href="blln-20250930.xsd#blln_SeriesA4RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_SeriesA4RedeemableConvertiblePreferredStockMember_200bdb60-07e7-4fb6-83ef-2d7a792f46fb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesA5RedeemableConvertiblePreferredStockMember_6c2f97f3-b2cd-4fbb-bb4a-e9c24318ecab" xlink:href="blln-20250930.xsd#blln_SeriesA5RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_SeriesA5RedeemableConvertiblePreferredStockMember_6c2f97f3-b2cd-4fbb-bb4a-e9c24318ecab" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesA6RedeemableConvertiblePreferredStockMember_b85ea13d-6760-47d4-a642-d429b3ff5b08" xlink:href="blln-20250930.xsd#blln_SeriesA6RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_SeriesA6RedeemableConvertiblePreferredStockMember_b85ea13d-6760-47d4-a642-d429b3ff5b08" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesB1RedeemableConvertiblePreferredStockMember_0b70fd9d-55f5-445c-8d24-1252d03b1b3e" xlink:href="blln-20250930.xsd#blln_SeriesB1RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_SeriesB1RedeemableConvertiblePreferredStockMember_0b70fd9d-55f5-445c-8d24-1252d03b1b3e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesB2RedeemableConvertiblePreferredStockMember_52280a6d-669d-4ad8-b1d5-810a00e3eec0" xlink:href="blln-20250930.xsd#blln_SeriesB2RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_SeriesB2RedeemableConvertiblePreferredStockMember_52280a6d-669d-4ad8-b1d5-810a00e3eec0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesCRedeemableConvertiblePreferredStockMember_7289be56-7434-49b6-b968-6081befe12ba" xlink:href="blln-20250930.xsd#blln_SeriesCRedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_SeriesCRedeemableConvertiblePreferredStockMember_7289be56-7434-49b6-b968-6081befe12ba" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesC1RedeemableConvertiblePreferredStockMember_566c844e-5673-4767-a680-b8e1e35cb5d4" xlink:href="blln-20250930.xsd#blln_SeriesC1RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_SeriesC1RedeemableConvertiblePreferredStockMember_566c844e-5673-4767-a680-b8e1e35cb5d4" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_SeriesDRedeemableConvertiblePreferredStockMember_13cae5fc-71e7-4154-93fa-31ede09c09b4" xlink:href="blln-20250930.xsd#blln_SeriesDRedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_SeriesDRedeemableConvertiblePreferredStockMember_13cae5fc-71e7-4154-93fa-31ede09c09b4" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_RedeemableConvertiblePreferredStockWarrantsMember_8eda15c7-bb71-4dac-a4ab-ee3d6fb688ea" xlink:href="blln-20250930.xsd#blln_RedeemableConvertiblePreferredStockWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_RedeemableConvertiblePreferredStockWarrantsMember_8eda15c7-bb71-4dac-a4ab-ee3d6fb688ea" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_CommonStockWarrantsMember_a9003c92-cd4e-4234-b237-3d96e046397c" xlink:href="blln-20250930.xsd#blln_CommonStockWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_CommonStockWarrantsMember_a9003c92-cd4e-4234-b237-3d96e046397c" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_OptionsToPurchaseCommonStockMember_ac663a58-908a-49a3-b0e7-66efd5ef3e8f" xlink:href="blln-20250930.xsd#blln_OptionsToPurchaseCommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_OptionsToPurchaseCommonStockMember_ac663a58-908a-49a3-b0e7-66efd5ef3e8f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_StockOptionsAvailableForFutureGrantsMember_cfea8dc9-9995-4618-882a-90b2dff2d172" xlink:href="blln-20250930.xsd#blln_StockOptionsAvailableForFutureGrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_8ccd8aff-5873-4578-b1e5-ef41bf6cc68e" xlink:to="loc_blln_StockOptionsAvailableForFutureGrantsMember_cfea8dc9-9995-4618-882a-90b2dff2d172" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_0bca95a4-dde4-4538-a68c-5cb20585ebe6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_4d294199-d2d8-4bb7-bcee-0fbf526efd3b" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_0bca95a4-dde4-4538-a68c-5cb20585ebe6" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/CommonStockValuationAssumptionsDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#CommonStockValuationAssumptionsDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/CommonStockValuationAssumptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6aeae840-62cf-498f-af35-6064ae56f2ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_a9a21444-5b52-43e4-8fac-6c3d1a002c0d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6aeae840-62cf-498f-af35-6064ae56f2ee" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_a9a21444-5b52-43e4-8fac-6c3d1a002c0d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_e3284a68-8e38-4cac-bbd7-7e8739ae2f79" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_a9a21444-5b52-43e4-8fac-6c3d1a002c0d" xlink:to="loc_us-gaap_AwardTypeAxis_e3284a68-8e38-4cac-bbd7-7e8739ae2f79" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e3284a68-8e38-4cac-bbd7-7e8739ae2f79_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_e3284a68-8e38-4cac-bbd7-7e8739ae2f79" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e3284a68-8e38-4cac-bbd7-7e8739ae2f79_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ba31f91b-5e15-4895-91c1-7d45962f26de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_e3284a68-8e38-4cac-bbd7-7e8739ae2f79" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ba31f91b-5e15-4895-91c1-7d45962f26de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_f56477dc-07ad-481c-a2f4-08d299b710b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ba31f91b-5e15-4895-91c1-7d45962f26de" xlink:to="loc_us-gaap_EmployeeStockOptionMember_f56477dc-07ad-481c-a2f4-08d299b710b7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsFairValueOfCommonStock_ef48140e-148a-489e-a2da-3e06f249a6d9" xlink:href="blln-20250930.xsd#blln_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsFairValueOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6aeae840-62cf-498f-af35-6064ae56f2ee" xlink:to="loc_blln_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsFairValueOfCommonStock_ef48140e-148a-489e-a2da-3e06f249a6d9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_c8dc1818-7b7b-4d9e-900e-998e9c87ead9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6aeae840-62cf-498f-af35-6064ae56f2ee" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_c8dc1818-7b7b-4d9e-900e-998e9c87ead9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_717b96c0-e3ad-4857-a52d-f4176d10a5f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6aeae840-62cf-498f-af35-6064ae56f2ee" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_717b96c0-e3ad-4857-a52d-f4176d10a5f3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate_3840945e-55bc-4abe-9307-0ab79a42687a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6aeae840-62cf-498f-af35-6064ae56f2ee" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate_3840945e-55bc-4abe-9307-0ab79a42687a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_afd5bcb4-626e-49c2-adea-2393a0baec86" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6aeae840-62cf-498f-af35-6064ae56f2ee" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_afd5bcb4-626e-49c2-adea-2393a0baec86" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/CommonStockStockBasedCompensationDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#CommonStockStockBasedCompensationDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/CommonStockStockBasedCompensationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_30967de9-c718-4d78-bcec-31039bddb708" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_28d24006-7fc0-4b05-997d-dad01fcc2d59" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_30967de9-c718-4d78-bcec-31039bddb708" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_28d24006-7fc0-4b05-997d-dad01fcc2d59" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_2a08ad38-af86-4834-9543-67c5469c869d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_28d24006-7fc0-4b05-997d-dad01fcc2d59" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_2a08ad38-af86-4834-9543-67c5469c869d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_2a08ad38-af86-4834-9543-67c5469c869d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_2a08ad38-af86-4834-9543-67c5469c869d" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_2a08ad38-af86-4834-9543-67c5469c869d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_77053fa1-8f47-4183-b27c-1330f1436e3c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_2a08ad38-af86-4834-9543-67c5469c869d" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_77053fa1-8f47-4183-b27c-1330f1436e3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_CostOfProductAndServiceSoldMember_018d201a-2a6c-4feb-8b8d-91d05eaef96a" xlink:href="blln-20250930.xsd#blln_CostOfProductAndServiceSoldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_77053fa1-8f47-4183-b27c-1330f1436e3c" xlink:to="loc_blln_CostOfProductAndServiceSoldMember_018d201a-2a6c-4feb-8b8d-91d05eaef96a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_81891ba0-a230-477f-8f49-9661c637b892" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_77053fa1-8f47-4183-b27c-1330f1436e3c" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_81891ba0-a230-477f-8f49-9661c637b892" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_9b10515f-3bd7-4c3f-87a0-0447888e9654" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_77053fa1-8f47-4183-b27c-1330f1436e3c" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_9b10515f-3bd7-4c3f-87a0-0447888e9654" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_94042ffe-fa3f-4a63-9165-053d50ea38d8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_30967de9-c718-4d78-bcec-31039bddb708" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_94042ffe-fa3f-4a63-9165-053d50ea38d8" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/RelatedPartyTransactionsDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#RelatedPartyTransactionsDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/RelatedPartyTransactionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_505416f8-68da-4e6a-a41f-d42b8148371c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_ce1c27c9-ae2c-41b1-a3c4-de30282294af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_505416f8-68da-4e6a-a41f-d42b8148371c" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_ce1c27c9-ae2c-41b1-a3c4-de30282294af" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_09d02280-6ee8-49ed-9498-12d91732eeb0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_ce1c27c9-ae2c-41b1-a3c4-de30282294af" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_09d02280-6ee8-49ed-9498-12d91732eeb0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_09d02280-6ee8-49ed-9498-12d91732eeb0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_09d02280-6ee8-49ed-9498-12d91732eeb0" xlink:to="loc_us-gaap_RelatedPartyDomain_09d02280-6ee8-49ed-9498-12d91732eeb0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_69b757d7-35f8-46ba-a1df-4e65563fd43d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_09d02280-6ee8-49ed-9498-12d91732eeb0" xlink:to="loc_us-gaap_RelatedPartyDomain_69b757d7-35f8-46ba-a1df-4e65563fd43d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_07f99a99-2647-4929-8ac6-b74c29e03797" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_69b757d7-35f8-46ba-a1df-4e65563fd43d" xlink:to="loc_us-gaap_RelatedPartyMember_07f99a99-2647-4929-8ac6-b74c29e03797" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_d867549e-432e-485b-8e8c-1c3d44a36ff7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_505416f8-68da-4e6a-a41f-d42b8148371c" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_d867549e-432e-485b-8e8c-1c3d44a36ff7" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/NetIncomeLossPerShareAttributabletoShareholdersComputationofBasicandDilutiveNetLossPerShareDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#NetIncomeLossPerShareAttributabletoShareholdersComputationofBasicandDilutiveNetLossPerShareDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/NetIncomeLossPerShareAttributabletoShareholdersComputationofBasicandDilutiveNetLossPerShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDilutedLineItems_febe6d31-b872-454b-b629-3a03e3a910f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDilutedLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable_bf1d8164-f2a9-4cbf-afcf-04fee2cea6fb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EarningsPerShareDilutedLineItems_febe6d31-b872-454b-b629-3a03e3a910f3" xlink:to="loc_us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable_bf1d8164-f2a9-4cbf-afcf-04fee2cea6fb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_630601cf-91e3-4c9f-8b8d-5101cd5d2e68" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable_bf1d8164-f2a9-4cbf-afcf-04fee2cea6fb" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_630601cf-91e3-4c9f-8b8d-5101cd5d2e68" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_630601cf-91e3-4c9f-8b8d-5101cd5d2e68_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_630601cf-91e3-4c9f-8b8d-5101cd5d2e68" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_630601cf-91e3-4c9f-8b8d-5101cd5d2e68_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_f393a43a-2700-4180-bd3b-0ea60ab46807" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_630601cf-91e3-4c9f-8b8d-5101cd5d2e68" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_f393a43a-2700-4180-bd3b-0ea60ab46807" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_RedeemableConvertiblePreferredStockWarrantsMember_c5e08a5a-dcf9-4bc0-9deb-2bd1953f3705" xlink:href="blln-20250930.xsd#blln_RedeemableConvertiblePreferredStockWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_f393a43a-2700-4180-bd3b-0ea60ab46807" xlink:to="loc_blln_RedeemableConvertiblePreferredStockWarrantsMember_c5e08a5a-dcf9-4bc0-9deb-2bd1953f3705" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_CommonStockWarrantsMember_58d09fd3-61c4-4bb8-984d-35e693e44f06" xlink:href="blln-20250930.xsd#blln_CommonStockWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_f393a43a-2700-4180-bd3b-0ea60ab46807" xlink:to="loc_blln_CommonStockWarrantsMember_58d09fd3-61c4-4bb8-984d-35e693e44f06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasicAbstract_b262831d-86bb-4542-a621-a9493874cfa3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasicAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedLineItems_febe6d31-b872-454b-b629-3a03e3a910f3" xlink:to="loc_us-gaap_EarningsPerShareBasicAbstract_b262831d-86bb-4542-a621-a9493874cfa3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract_6b275a25-a883-4627-a0d4-ac884b6ab1b6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_b262831d-86bb-4542-a621-a9493874cfa3" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract_6b275a25-a883-4627-a0d4-ac884b6ab1b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_c7c75da8-d238-4d81-b238-5edfd9245695" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract_6b275a25-a883-4627-a0d4-ac884b6ab1b6" xlink:to="loc_us-gaap_NetIncomeLoss_c7c75da8-d238-4d81-b238-5edfd9245695" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ParticipatingSecuritiesDistributedAndUndistributedEarningsLossBasic_b6ee82a8-4506-479b-a61e-1c5883e76b1f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ParticipatingSecuritiesDistributedAndUndistributedEarningsLossBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract_6b275a25-a883-4627-a0d4-ac884b6ab1b6" xlink:to="loc_us-gaap_ParticipatingSecuritiesDistributedAndUndistributedEarningsLossBasic_b6ee82a8-4506-479b-a61e-1c5883e76b1f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_de849d9d-85cb-45e4-ac45-b8d221e38f66" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract_6b275a25-a883-4627-a0d4-ac884b6ab1b6" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_de849d9d-85cb-45e4-ac45-b8d221e38f66" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_9fef5502-0342-41ce-87dd-9a759aa46b86" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_b262831d-86bb-4542-a621-a9493874cfa3" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_9fef5502-0342-41ce-87dd-9a759aa46b86" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_d201a499-64fe-4983-8260-eeefacbcf7d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_9fef5502-0342-41ce-87dd-9a759aa46b86" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_d201a499-64fe-4983-8260-eeefacbcf7d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_7f405fb7-42c5-4bcc-a39f-28201c16a92f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_9fef5502-0342-41ce-87dd-9a759aa46b86" xlink:to="loc_us-gaap_EarningsPerShareBasic_7f405fb7-42c5-4bcc-a39f-28201c16a92f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDilutedAbstract_59dde741-ab0f-4992-ac9b-19e1e52942dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedLineItems_febe6d31-b872-454b-b629-3a03e3a910f3" xlink:to="loc_us-gaap_EarningsPerShareDilutedAbstract_59dde741-ab0f-4992-ac9b-19e1e52942dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_6bc253e6-03c1-449f-83e2-b993f3244920" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_59dde741-ab0f-4992-ac9b-19e1e52942dd" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_6bc253e6-03c1-449f-83e2-b993f3244920" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_e7fb6018-5238-402d-a608-d93e26bfd39e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_6bc253e6-03c1-449f-83e2-b993f3244920" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_e7fb6018-5238-402d-a608-d93e26bfd39e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_f65d53d9-8710-4fe6-b86e-2784d7c6d6ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_59dde741-ab0f-4992-ac9b-19e1e52942dd" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_f65d53d9-8710-4fe6-b86e-2784d7c6d6ce" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_1d6d1370-4e71-4ed4-ace4-968d2f4175d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_f65d53d9-8710-4fe6-b86e-2784d7c6d6ce" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_1d6d1370-4e71-4ed4-ace4-968d2f4175d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToCallOptionsAndWarrants_9ef430c6-3b27-417e-80c7-75fe50499807" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncrementalCommonSharesAttributableToCallOptionsAndWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_f65d53d9-8710-4fe6-b86e-2784d7c6d6ce" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToCallOptionsAndWarrants_9ef430c6-3b27-417e-80c7-75fe50499807" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities_d5cb29fb-f4aa-49e1-933b-e2830e5cc8d8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_f65d53d9-8710-4fe6-b86e-2784d7c6d6ce" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities_d5cb29fb-f4aa-49e1-933b-e2830e5cc8d8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_be3954af-8b56-42cf-be1d-efea401f48d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_f65d53d9-8710-4fe6-b86e-2784d7c6d6ce" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_be3954af-8b56-42cf-be1d-efea401f48d9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_afb1c715-c106-48e1-bd81-13e0228e2381" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedLineItems_febe6d31-b872-454b-b629-3a03e3a910f3" xlink:to="loc_us-gaap_EarningsPerShareDiluted_afb1c715-c106-48e1-bd81-13e0228e2381" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/NetIncomeLossPerShareAttributabletoShareholdersPotentiallyDilutiveSecuritiesExcludedFromComputationDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#NetIncomeLossPerShareAttributabletoShareholdersPotentiallyDilutiveSecuritiesExcludedFromComputationDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/NetIncomeLossPerShareAttributabletoShareholdersPotentiallyDilutiveSecuritiesExcludedFromComputationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_cafc7682-e540-409f-8181-81bf8ed15d25" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_8b27698f-a457-4edc-a581-ba9fe9525e3b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_cafc7682-e540-409f-8181-81bf8ed15d25" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_8b27698f-a457-4edc-a581-ba9fe9525e3b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_a3a40b99-ab95-434c-81c9-97ff163374b4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_8b27698f-a457-4edc-a581-ba9fe9525e3b" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_a3a40b99-ab95-434c-81c9-97ff163374b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_a3a40b99-ab95-434c-81c9-97ff163374b4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_a3a40b99-ab95-434c-81c9-97ff163374b4" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_a3a40b99-ab95-434c-81c9-97ff163374b4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_58dc0560-1092-4138-ba32-65a5d56f7429" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_a3a40b99-ab95-434c-81c9-97ff163374b4" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_58dc0560-1092-4138-ba32-65a5d56f7429" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RedeemableConvertiblePreferredStockMember_ea7ac8b1-65e9-4494-96ff-d2296a953576" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_58dc0560-1092-4138-ba32-65a5d56f7429" xlink:to="loc_us-gaap_RedeemableConvertiblePreferredStockMember_ea7ac8b1-65e9-4494-96ff-d2296a953576" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_44f8ec04-6d95-4728-9730-caf61161c167" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_58dc0560-1092-4138-ba32-65a5d56f7429" xlink:to="loc_us-gaap_EmployeeStockOptionMember_44f8ec04-6d95-4728-9730-caf61161c167" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_3b9fc10f-f69f-4663-951e-c53f2c52d006" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_58dc0560-1092-4138-ba32-65a5d56f7429" xlink:to="loc_us-gaap_RestrictedStockMember_3b9fc10f-f69f-4663-951e-c53f2c52d006" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_AntidilutiveSecuritiesWarrantsToPurchaseCommonStockMember_596c7fc3-335f-4c51-9f20-f7181975b0f8" xlink:href="blln-20250930.xsd#blln_AntidilutiveSecuritiesWarrantsToPurchaseCommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_58dc0560-1092-4138-ba32-65a5d56f7429" xlink:to="loc_blln_AntidilutiveSecuritiesWarrantsToPurchaseCommonStockMember_596c7fc3-335f-4c51-9f20-f7181975b0f8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_AntidilutiveSecuritiesWarrantsToPurchaseRedeemableConvertiblePreferredStockMember_1a407226-ffe7-46af-9e23-14fdef8b9cfb" xlink:href="blln-20250930.xsd#blln_AntidilutiveSecuritiesWarrantsToPurchaseRedeemableConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_58dc0560-1092-4138-ba32-65a5d56f7429" xlink:to="loc_blln_AntidilutiveSecuritiesWarrantsToPurchaseRedeemableConvertiblePreferredStockMember_1a407226-ffe7-46af-9e23-14fdef8b9cfb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_AntidilutiveSecuritiesUnvestedEarlyExercisedOptionsMember_d9ad4f05-e981-493b-8cdb-f49ff3adbecb" xlink:href="blln-20250930.xsd#blln_AntidilutiveSecuritiesUnvestedEarlyExercisedOptionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_58dc0560-1092-4138-ba32-65a5d56f7429" xlink:to="loc_blln_AntidilutiveSecuritiesUnvestedEarlyExercisedOptionsMember_d9ad4f05-e981-493b-8cdb-f49ff3adbecb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_da0bb539-4d74-48a2-9b7d-c6347d5a363a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_cafc7682-e540-409f-8181-81bf8ed15d25" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_da0bb539-4d74-48a2-9b7d-c6347d5a363a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.billiontoone.com/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="blln-20250930.xsd#SubsequentEventsDetails"/>
  <link:definitionLink xlink:role="http://www.billiontoone.com/role/SubsequentEventsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_c099e3d0-1e85-46b4-944e-d40f44cb1df4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_b2e74ff0-f2be-47b7-96c3-516010ee4fc3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_c099e3d0-1e85-46b4-944e-d40f44cb1df4" xlink:to="loc_us-gaap_SubsequentEventTable_b2e74ff0-f2be-47b7-96c3-516010ee4fc3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_f958888d-c94f-4944-b047-39d479565f57" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_b2e74ff0-f2be-47b7-96c3-516010ee4fc3" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_f958888d-c94f-4944-b047-39d479565f57" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_f958888d-c94f-4944-b047-39d479565f57_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_f958888d-c94f-4944-b047-39d479565f57" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_f958888d-c94f-4944-b047-39d479565f57_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_bb8e022d-d18e-4b3e-ab15-03a207a91454" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_f958888d-c94f-4944-b047-39d479565f57" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_bb8e022d-d18e-4b3e-ab15-03a207a91454" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IPOMember_90a4c7cd-c54e-4802-b64f-dae528dff42e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IPOMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_bb8e022d-d18e-4b3e-ab15-03a207a91454" xlink:to="loc_us-gaap_IPOMember_90a4c7cd-c54e-4802-b64f-dae528dff42e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OverAllotmentOptionMember_a225b195-4f54-4df4-b5e7-1003a36f6b5a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OverAllotmentOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_bb8e022d-d18e-4b3e-ab15-03a207a91454" xlink:to="loc_us-gaap_OverAllotmentOptionMember_a225b195-4f54-4df4-b5e7-1003a36f6b5a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_4aae818e-9208-4dc6-aeb9-5967a3e9d66d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_b2e74ff0-f2be-47b7-96c3-516010ee4fc3" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_4aae818e-9208-4dc6-aeb9-5967a3e9d66d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_4aae818e-9208-4dc6-aeb9-5967a3e9d66d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_4aae818e-9208-4dc6-aeb9-5967a3e9d66d" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_4aae818e-9208-4dc6-aeb9-5967a3e9d66d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_a29fa884-88df-4455-a10c-78c21e0f959c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_4aae818e-9208-4dc6-aeb9-5967a3e9d66d" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_a29fa884-88df-4455-a10c-78c21e0f959c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_f504f314-4a15-4cfd-b1b7-e10169d1204c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_a29fa884-88df-4455-a10c-78c21e0f959c" xlink:to="loc_us-gaap_SubsequentEventMember_f504f314-4a15-4cfd-b1b7-e10169d1204c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_af482fec-244a-40f9-a4ad-fbc9bb16b918" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_b2e74ff0-f2be-47b7-96c3-516010ee4fc3" xlink:to="loc_us-gaap_StatementClassOfStockAxis_af482fec-244a-40f9-a4ad-fbc9bb16b918" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_af482fec-244a-40f9-a4ad-fbc9bb16b918_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_af482fec-244a-40f9-a4ad-fbc9bb16b918" xlink:to="loc_us-gaap_ClassOfStockDomain_af482fec-244a-40f9-a4ad-fbc9bb16b918_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_bf07bd17-f36b-461a-a0fc-43952875cad8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_af482fec-244a-40f9-a4ad-fbc9bb16b918" xlink:to="loc_us-gaap_ClassOfStockDomain_bf07bd17-f36b-461a-a0fc-43952875cad8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_9aab4e1c-0873-4553-be4b-7e913ca88f34" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_bf07bd17-f36b-461a-a0fc-43952875cad8" xlink:to="loc_us-gaap_CommonClassAMember_9aab4e1c-0873-4553-be4b-7e913ca88f34" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_019afef0-37df-7649-aba2-ba7672577d26" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_bf07bd17-f36b-461a-a0fc-43952875cad8" xlink:to="loc_us-gaap_CommonClassBMember_019afef0-37df-7649-aba2-ba7672577d26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramAxis_816aedea-740b-461b-82b7-892dd7c3fa2d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_b2e74ff0-f2be-47b7-96c3-516010ee4fc3" xlink:to="loc_srt_ShareRepurchaseProgramAxis_816aedea-740b-461b-82b7-892dd7c3fa2d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_816aedea-740b-461b-82b7-892dd7c3fa2d_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ShareRepurchaseProgramAxis_816aedea-740b-461b-82b7-892dd7c3fa2d" xlink:to="loc_srt_ShareRepurchaseProgramDomain_816aedea-740b-461b-82b7-892dd7c3fa2d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_71d4f3d0-c05b-4ee0-9c6c-efe8963abb5a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ShareRepurchaseProgramAxis_816aedea-740b-461b-82b7-892dd7c3fa2d" xlink:to="loc_srt_ShareRepurchaseProgramDomain_71d4f3d0-c05b-4ee0-9c6c-efe8963abb5a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_The2025PlanMember_8fafad57-6a17-4ba1-b9f6-aa682ebe4e8e" xlink:href="blln-20250930.xsd#blln_The2025PlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramDomain_71d4f3d0-c05b-4ee0-9c6c-efe8963abb5a" xlink:to="loc_blln_The2025PlanMember_8fafad57-6a17-4ba1-b9f6-aa682ebe4e8e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_The2018PlanMember_aa2c923c-9a4c-4299-8376-0e41a98bb13d" xlink:href="blln-20250930.xsd#blln_The2018PlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramDomain_71d4f3d0-c05b-4ee0-9c6c-efe8963abb5a" xlink:to="loc_blln_The2018PlanMember_aa2c923c-9a4c-4299-8376-0e41a98bb13d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_73b4e1c9-c8e3-413b-87dc-5565bf6703a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_b2e74ff0-f2be-47b7-96c3-516010ee4fc3" xlink:to="loc_us-gaap_AwardTypeAxis_73b4e1c9-c8e3-413b-87dc-5565bf6703a1" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_73b4e1c9-c8e3-413b-87dc-5565bf6703a1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_73b4e1c9-c8e3-413b-87dc-5565bf6703a1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_73b4e1c9-c8e3-413b-87dc-5565bf6703a1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_804f4bb6-b2fb-4b64-ac4a-fcc580ccf10c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_73b4e1c9-c8e3-413b-87dc-5565bf6703a1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_804f4bb6-b2fb-4b64-ac4a-fcc580ccf10c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_557e4f8f-fbdf-4045-a522-408d1fcc7690" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_804f4bb6-b2fb-4b64-ac4a-fcc580ccf10c" xlink:to="loc_us-gaap_EmployeeStockMember_557e4f8f-fbdf-4045-a522-408d1fcc7690" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019afef0-cc52-7a1a-a5c1-6f248ef70224" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_b2e74ff0-f2be-47b7-96c3-516010ee4fc3" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019afef0-cc52-7a1a-a5c1-6f248ef70224" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019afef0-cc52-7a1a-a5c1-6f248ef70224_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019afef0-cc52-7a1a-a5c1-6f248ef70224" xlink:to="loc_us-gaap_RelatedPartyDomain_019afef0-cc52-7a1a-a5c1-6f248ef70224_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019afef0-cc52-7008-99d7-36b3af443a02" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019afef0-cc52-7a1a-a5c1-6f248ef70224" xlink:to="loc_us-gaap_RelatedPartyDomain_019afef0-cc52-7008-99d7-36b3af443a02" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ChiefExecutiveOfficerMember_019afef1-3912-7e6e-b7f1-cb631628a635" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ChiefExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019afef0-cc52-7008-99d7-36b3af443a02" xlink:to="loc_srt_ChiefExecutiveOfficerMember_019afef1-3912-7e6e-b7f1-cb631628a635" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ExecutiveOfficerMember_019afef1-84e1-7ea1-9b62-0d4da0710a1e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019afef0-cc52-7008-99d7-36b3af443a02" xlink:to="loc_srt_ExecutiveOfficerMember_019afef1-84e1-7ea1-9b62-0d4da0710a1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_b5214865-31b2-4e9a-bdd4-d67074b7324f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_c099e3d0-1e85-46b4-944e-d40f44cb1df4" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_b5214865-31b2-4e9a-bdd4-d67074b7324f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_05179c67-9d4e-4b47-91d3-f67dfd46f8bc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_c099e3d0-1e85-46b4-944e-d40f44cb1df4" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_05179c67-9d4e-4b47-91d3-f67dfd46f8bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_0e8b3268-5c67-45de-940d-23229ab37fa6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_c099e3d0-1e85-46b4-944e-d40f44cb1df4" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_0e8b3268-5c67-45de-940d-23229ab37fa6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_e8b30953-f3a9-4906-82cb-638f3fc6470e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_c099e3d0-1e85-46b4-944e-d40f44cb1df4" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_e8b30953-f3a9-4906-82cb-638f3fc6470e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_61dece45-5200-4eb3-af36-4561a7469564" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_c099e3d0-1e85-46b4-944e-d40f44cb1df4" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_61dece45-5200-4eb3-af36-4561a7469564" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingNumberOfSharesAuthorizedToBeRepurchased_c1448766-6ed3-4160-b859-2f2c34f58664" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchaseProgramRemainingNumberOfSharesAuthorizedToBeRepurchased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_c099e3d0-1e85-46b4-944e-d40f44cb1df4" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingNumberOfSharesAuthorizedToBeRepurchased_c1448766-6ed3-4160-b859-2f2c34f58664" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_a9f83c05-c472-4318-9d57-5a0e3660b4d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_c099e3d0-1e85-46b4-944e-d40f44cb1df4" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_a9f83c05-c472-4318-9d57-5a0e3660b4d9" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_4c923e9d-de79-4f84-930e-893aa17af0d2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_c099e3d0-1e85-46b4-944e-d40f44cb1df4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_4c923e9d-de79-4f84-930e-893aa17af0d2" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_blln_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorizedPerParticipant_5f1164ad-36b5-47dc-885f-c08876469a5d" xlink:href="blln-20250930.xsd#blln_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorizedPerParticipant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_c099e3d0-1e85-46b4-944e-d40f44cb1df4" xlink:to="loc_blln_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorizedPerParticipant_5f1164ad-36b5-47dc-885f-c08876469a5d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_eae9b696-cd21-4716-876f-f7772d1bd59d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_c099e3d0-1e85-46b4-944e-d40f44cb1df4" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_eae9b696-cd21-4716-876f-f7772d1bd59d" xlink:type="arc" order="9"/>
  </link:definitionLink>
</link:linkbase>
