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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:01a020bc-05d6-7326-aa3c-c583ed6e9c2b,g:a7a26c87-2037-4424-9631-c46bc6606ffb-->
<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.acumenpharm.com/role/CondensedStatementsofChangesinStockholdersEquity" xlink:type="simple" xlink:href="abos-20260930.xsd#CondensedStatementsofChangesinStockholdersEquity"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/CondensedStatementsofChangesinStockholdersEquity" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_01a020bc-066a-7901-a650-cf2e73a2b286" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_01a020bc-066a-7287-a94f-b1e0f24c95ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_01a020bc-066a-7901-a650-cf2e73a2b286" xlink:to="loc_us-gaap_StatementTable_01a020bc-066a-7287-a94f-b1e0f24c95ff" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_01a020bc-066a-70c5-ba49-307e031cc925" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_01a020bc-066a-7287-a94f-b1e0f24c95ff" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_01a020bc-066a-70c5-ba49-307e031cc925" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a020bc-066a-70c5-ba49-307e031cc925_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_01a020bc-066a-70c5-ba49-307e031cc925" xlink:to="loc_us-gaap_EquityComponentDomain_01a020bc-066a-70c5-ba49-307e031cc925_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a020bc-066a-7033-a0ae-e1e7bd68e78d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_01a020bc-066a-70c5-ba49-307e031cc925" xlink:to="loc_us-gaap_EquityComponentDomain_01a020bc-066a-7033-a0ae-e1e7bd68e78d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_01a020bc-066a-7e2f-ac89-116088f19d3d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a020bc-066a-7033-a0ae-e1e7bd68e78d" xlink:to="loc_us-gaap_CommonStockMember_01a020bc-066a-7e2f-ac89-116088f19d3d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_01a020bc-066a-7a53-b94b-edf3759708a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a020bc-066a-7033-a0ae-e1e7bd68e78d" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_01a020bc-066a-7a53-b94b-edf3759708a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_01a020bc-066a-77ad-bf94-91f86a26654e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a020bc-066a-7033-a0ae-e1e7bd68e78d" xlink:to="loc_us-gaap_RetainedEarningsMember_01a020bc-066a-77ad-bf94-91f86a26654e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_01a020bc-066a-7e77-879c-ac5e9420a22f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a020bc-066a-7033-a0ae-e1e7bd68e78d" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_01a020bc-066a-7e77-879c-ac5e9420a22f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a020bc-066a-73d6-9ab2-f0c8d832dc57" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a020bc-066a-7901-a650-cf2e73a2b286" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a020bc-066a-73d6-9ab2-f0c8d832dc57" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_01a020bc-066a-72cc-baf9-ab99d2e49fee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a020bc-066a-73d6-9ab2-f0c8d832dc57" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_01a020bc-066a-72cc-baf9-ab99d2e49fee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_01a020bc-066a-7bbb-9838-d88129c73ffe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a020bc-066a-73d6-9ab2-f0c8d832dc57" xlink:to="loc_us-gaap_StockholdersEquity_01a020bc-066a-7bbb-9838-d88129c73ffe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_01a020bc-066a-72d7-baa3-905f4a9a7e04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a020bc-066a-73d6-9ab2-f0c8d832dc57" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_01a020bc-066a-72d7-baa3-905f4a9a7e04" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_01a020bc-066a-7078-b6a3-4a6236aaca6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a020bc-066a-73d6-9ab2-f0c8d832dc57" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_01a020bc-066a-7078-b6a3-4a6236aaca6f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockSharesIssuedNetOfSharesForTaxWithholdings_01a020bc-066a-797d-8e18-366e857a2cd5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockSharesIssuedNetOfSharesForTaxWithholdings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a020bc-066a-73d6-9ab2-f0c8d832dc57" xlink:to="loc_us-gaap_RestrictedStockSharesIssuedNetOfSharesForTaxWithholdings_01a020bc-066a-797d-8e18-366e857a2cd5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockValueSharesIssuedNetOfTaxWithholdings_01a020bc-066a-7ffe-ab78-3a392ae61023" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockValueSharesIssuedNetOfTaxWithholdings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a020bc-066a-73d6-9ab2-f0c8d832dc57" xlink:to="loc_us-gaap_RestrictedStockValueSharesIssuedNetOfTaxWithholdings_01a020bc-066a-7ffe-ab78-3a392ae61023" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_01a020bc-066a-7285-85a9-84ac79aa3b48" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a020bc-066a-73d6-9ab2-f0c8d832dc57" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_01a020bc-066a-7285-85a9-84ac79aa3b48" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_01a020bc-066a-7a89-b34b-a99c6022c83c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a020bc-066a-73d6-9ab2-f0c8d832dc57" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_01a020bc-066a-7a89-b34b-a99c6022c83c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax_01a020bc-066a-7158-9f07-1eca57e90b71" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a020bc-066a-73d6-9ab2-f0c8d832dc57" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax_01a020bc-066a-7158-9f07-1eca57e90b71" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_01a020bc-066a-7320-9a4a-7f6098e79e19" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a020bc-066a-73d6-9ab2-f0c8d832dc57" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_01a020bc-066a-7320-9a4a-7f6098e79e19" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_01a020bc-066a-7913-92c7-d54f0a3813a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a020bc-066a-73d6-9ab2-f0c8d832dc57" xlink:to="loc_us-gaap_NetIncomeLoss_01a020bc-066a-7913-92c7-d54f0a3813a1" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_01a020bc-066a-73bf-8f37-4fb367d69188" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_01a020bc-066a-7d60-b40e-c9ba22540743" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofAntidilutiveSecuritiesExcludedfromComputationofLossPerShareDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofAntidilutiveSecuritiesExcludedfromComputationofLossPerShareDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/BASISOFPRESENTATIONANDSUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofAntidilutiveSecuritiesExcludedfromComputationofLossPerShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_01a020bc-066b-709f-a137-9c061fd7ef7b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_01a020bc-066b-702a-bb2f-ad579a23cf89" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_01a020bc-066b-709f-a137-9c061fd7ef7b" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_01a020bc-066b-702a-bb2f-ad579a23cf89" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_01a020bc-066b-7b7b-b868-912ca26579d6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_01a020bc-066b-702a-bb2f-ad579a23cf89" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_01a020bc-066b-7b7b-b868-912ca26579d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a020bc-066b-7b7b-b868-912ca26579d6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_01a020bc-066b-7b7b-b868-912ca26579d6" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a020bc-066b-7b7b-b868-912ca26579d6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a020bc-066b-7b33-95ab-a5c348a6f6c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_01a020bc-066b-7b7b-b868-912ca26579d6" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a020bc-066b-7b33-95ab-a5c348a6f6c3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_01a020bc-066b-7327-9e76-08ae8e2c30ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a020bc-066b-7b33-95ab-a5c348a6f6c3" xlink:to="loc_us-gaap_EmployeeStockOptionMember_01a020bc-066b-7327-9e76-08ae8e2c30ec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtSecuritiesMember_01a020bc-066b-73cf-8416-c3a653a1c970" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a020bc-066b-7b33-95ab-a5c348a6f6c3" xlink:to="loc_us-gaap_ConvertibleDebtSecuritiesMember_01a020bc-066b-73cf-8416-c3a653a1c970" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_01a020bc-066b-7b49-9520-060877bd8ca5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a020bc-066b-7b33-95ab-a5c348a6f6c3" xlink:to="loc_us-gaap_WarrantMember_01a020bc-066b-7b49-9520-060877bd8ca5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_01a020bc-066b-7f34-951d-4cb11c3031a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a020bc-066b-7b33-95ab-a5c348a6f6c3" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_01a020bc-066b-7f34-951d-4cb11c3031a5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_01a020bc-066b-7a49-b737-94752c94afce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_01a020bc-066b-709f-a137-9c061fd7ef7b" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_01a020bc-066b-7a49-b737-94752c94afce" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/MARKETABLESECURITIESScheduleofMarketableSecuritiesDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#MARKETABLESECURITIESScheduleofMarketableSecuritiesDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/MARKETABLESECURITIESScheduleofMarketableSecuritiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesLineItems_01a020bc-066b-7268-98b8-8bef47c4ee0b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MarketableSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesTable_01a020bc-066b-783b-b7db-c967db9f5278" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MarketableSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_01a020bc-066b-7268-98b8-8bef47c4ee0b" xlink:to="loc_us-gaap_MarketableSecuritiesTable_01a020bc-066b-783b-b7db-c967db9f5278" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_01a020bc-066b-7991-9ff9-d0e74cef467d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_01a020bc-066b-783b-b7db-c967db9f5278" xlink:to="loc_us-gaap_FinancialInstrumentAxis_01a020bc-066b-7991-9ff9-d0e74cef467d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-7991-9ff9-d0e74cef467d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_01a020bc-066b-7991-9ff9-d0e74cef467d" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-7991-9ff9-d0e74cef467d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-7ba4-bc0f-8f89a4a7a021" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_01a020bc-066b-7991-9ff9-d0e74cef467d" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-7ba4-bc0f-8f89a4a7a021" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_01a020bc-066b-72f7-b4a5-896cf4ef46c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-7ba4-bc0f-8f89a4a7a021" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_01a020bc-066b-72f7-b4a5-896cf4ef46c7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_01a020bc-066b-78f1-98e0-78d503d74c50" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-7ba4-bc0f-8f89a4a7a021" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_01a020bc-066b-78f1-98e0-78d503d74c50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableforsaleAmortizedCostAbstract_01a020bc-066b-7404-b8b5-8cce4c1f22f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableforsaleAmortizedCostAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_01a020bc-066b-7268-98b8-8bef47c4ee0b" xlink:to="loc_us-gaap_DebtSecuritiesAvailableforsaleAmortizedCostAbstract_01a020bc-066b-7404-b8b5-8cce4c1f22f5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostCurrent_01a020bc-066b-7dd0-8f94-1300577bb147" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableforsaleAmortizedCostAbstract_01a020bc-066b-7404-b8b5-8cce4c1f22f5" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostCurrent_01a020bc-066b-7dd0-8f94-1300577bb147" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTaxAbstract_01a020bc-066b-720e-83c6-b8ad585b53de" xlink:href="abos-20260930.xsd#abos_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTaxAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_01a020bc-066b-7268-98b8-8bef47c4ee0b" xlink:to="loc_abos_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTaxAbstract_01a020bc-066b-720e-83c6-b8ad585b53de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTaxCurrent_01a020bc-066b-719d-b4b5-13266f0ca32f" xlink:href="abos-20260930.xsd#abos_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTaxCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_abos_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTaxAbstract_01a020bc-066b-720e-83c6-b8ad585b53de" xlink:to="loc_abos_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTaxCurrent_01a020bc-066b-719d-b4b5-13266f0ca32f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTaxAbstract_01a020bc-066b-76ab-be50-75a20af2f41f" xlink:href="abos-20260930.xsd#abos_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTaxAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_01a020bc-066b-7268-98b8-8bef47c4ee0b" xlink:to="loc_abos_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTaxAbstract_01a020bc-066b-76ab-be50-75a20af2f41f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTaxCurrent_01a020bc-066b-7155-a79a-5fb9231375b4" xlink:href="abos-20260930.xsd#abos_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTaxCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_abos_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTaxAbstract_01a020bc-066b-76ab-be50-75a20af2f41f" xlink:to="loc_abos_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTaxCurrent_01a020bc-066b-7155-a79a-5fb9231375b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesAbstract_01a020bc-066b-7380-b04b-0307bd5ae8fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_01a020bc-066b-7268-98b8-8bef47c4ee0b" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesAbstract_01a020bc-066b-7380-b04b-0307bd5ae8fe" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_01a020bc-066b-76a1-9b6d-845f776c4451" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_01a020bc-066b-7380-b04b-0307bd5ae8fe" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_01a020bc-066b-76a1-9b6d-845f776c4451" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/MARKETABLESECURITIESScheduleofUnrealizedLossonInvestmentsDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#MARKETABLESECURITIESScheduleofUnrealizedLossonInvestmentsDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/MARKETABLESECURITIESScheduleofUnrealizedLossonInvestmentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesLineItems_01a020bc-066b-7498-8366-48967706eca3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MarketableSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesTable_01a020bc-066b-7313-9d3c-79755070be51" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MarketableSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_01a020bc-066b-7498-8366-48967706eca3" xlink:to="loc_us-gaap_MarketableSecuritiesTable_01a020bc-066b-7313-9d3c-79755070be51" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_01a020bc-066b-72c3-90c0-5d0b62d29ab4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_MarketableSecuritiesTable_01a020bc-066b-7313-9d3c-79755070be51" xlink:to="loc_us-gaap_FinancialInstrumentAxis_01a020bc-066b-72c3-90c0-5d0b62d29ab4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-72c3-90c0-5d0b62d29ab4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_01a020bc-066b-72c3-90c0-5d0b62d29ab4" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-72c3-90c0-5d0b62d29ab4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-7164-aa1c-78dbd48d5b6a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_01a020bc-066b-72c3-90c0-5d0b62d29ab4" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-7164-aa1c-78dbd48d5b6a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_01a020bc-066b-7331-b6c6-f65cf588739f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-7164-aa1c-78dbd48d5b6a" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_01a020bc-066b-7331-b6c6-f65cf588739f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_01a020bc-066b-763a-bd56-02f3c84c67d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-7164-aa1c-78dbd48d5b6a" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_01a020bc-066b-763a-bd56-02f3c84c67d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAbstract_01a020bc-066b-785a-928d-f96ab0343658" xlink:href="abos-20260930.xsd#abos_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_01a020bc-066b-7498-8366-48967706eca3" xlink:to="loc_abos_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAbstract_01a020bc-066b-785a-928d-f96ab0343658" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_01a020bc-066b-7b8d-ad7d-f8476a74a469" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_abos_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAbstract_01a020bc-066b-785a-928d-f96ab0343658" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_01a020bc-066b-7b8d-ad7d-f8476a74a469" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_01a020bc-066b-776f-b565-3255f9c95b5b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_abos_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAbstract_01a020bc-066b-785a-928d-f96ab0343658" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_01a020bc-066b-776f-b565-3255f9c95b5b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAbstract_01a020bc-066b-790c-b091-eb2dba237b63" xlink:href="abos-20260930.xsd#abos_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_01a020bc-066b-7498-8366-48967706eca3" xlink:to="loc_abos_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAbstract_01a020bc-066b-790c-b091-eb2dba237b63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_01a020bc-066b-7776-a5fa-745238cec339" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_abos_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAbstract_01a020bc-066b-790c-b091-eb2dba237b63" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_01a020bc-066b-7776-a5fa-745238cec339" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_01a020bc-066b-71c8-a790-35149461a07a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_abos_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAbstract_01a020bc-066b-790c-b091-eb2dba237b63" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_01a020bc-066b-71c8-a790-35149461a07a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract_01a020bc-066b-7a0e-a0e2-f39286ba4187" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesLineItems_01a020bc-066b-7498-8366-48967706eca3" xlink:to="loc_us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract_01a020bc-066b-7a0e-a0e2-f39286ba4187" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_01a020bc-066b-7856-be2c-70a72925285f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract_01a020bc-066b-7a0e-a0e2-f39286ba4187" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_01a020bc-066b-7856-be2c-70a72925285f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_01a020bc-066b-74f9-bc73-9493e33b2583" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract_01a020bc-066b-7a0e-a0e2-f39286ba4187" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_01a020bc-066b-74f9-bc73-9493e33b2583" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/FAIRVALUEMEASUREMENTSScheduleofFairValueAssetsandLiabilitiesMeasuredonRecurringBasisDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#FAIRVALUEMEASUREMENTSScheduleofFairValueAssetsandLiabilitiesMeasuredonRecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/FAIRVALUEMEASUREMENTSScheduleofFairValueAssetsandLiabilitiesMeasuredonRecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a020bc-066b-76df-9dd6-c8bf8ed82f0f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a020bc-066b-73bd-aaad-a65e5c6ad2c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a020bc-066b-76df-9dd6-c8bf8ed82f0f" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a020bc-066b-73bd-aaad-a65e5c6ad2c4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_01a020bc-066b-7c15-8bfd-e8748dc71bcb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a020bc-066b-73bd-aaad-a65e5c6ad2c4" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_01a020bc-066b-7c15-8bfd-e8748dc71bcb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a020bc-066b-7c15-8bfd-e8748dc71bcb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_01a020bc-066b-7c15-8bfd-e8748dc71bcb" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a020bc-066b-7c15-8bfd-e8748dc71bcb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a020bc-066b-7e87-ba45-1c8abd6396bf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_01a020bc-066b-7c15-8bfd-e8748dc71bcb" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a020bc-066b-7e87-ba45-1c8abd6396bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_01a020bc-066b-744a-a079-ddbf4a1c3c56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a020bc-066b-7e87-ba45-1c8abd6396bf" xlink:to="loc_us-gaap_MoneyMarketFundsMember_01a020bc-066b-744a-a079-ddbf4a1c3c56" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_01a020bc-066b-7663-ae3b-9ebeeca457d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a020bc-066b-73bd-aaad-a65e5c6ad2c4" xlink:to="loc_us-gaap_FinancialInstrumentAxis_01a020bc-066b-7663-ae3b-9ebeeca457d3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-7663-ae3b-9ebeeca457d3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_01a020bc-066b-7663-ae3b-9ebeeca457d3" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-7663-ae3b-9ebeeca457d3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-70e6-96ac-0585ed0d00d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_01a020bc-066b-7663-ae3b-9ebeeca457d3" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-70e6-96ac-0585ed0d00d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_01a020bc-066b-7e4d-8aae-ef9bb1429ac6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-70e6-96ac-0585ed0d00d4" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_01a020bc-066b-7e4d-8aae-ef9bb1429ac6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_01a020bc-066b-7771-8835-c81ae0c0c132" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01a020bc-066b-70e6-96ac-0585ed0d00d4" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_01a020bc-066b-7771-8835-c81ae0c0c132" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a020bc-066b-758b-a168-49ae8ed58f9a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a020bc-066b-73bd-aaad-a65e5c6ad2c4" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a020bc-066b-758b-a168-49ae8ed58f9a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a020bc-066b-758b-a168-49ae8ed58f9a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a020bc-066b-758b-a168-49ae8ed58f9a" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a020bc-066b-758b-a168-49ae8ed58f9a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a020bc-066b-7f85-9447-11ab3aa854dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a020bc-066b-758b-a168-49ae8ed58f9a" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a020bc-066b-7f85-9447-11ab3aa854dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_01a020bc-066b-719d-b58f-0755001a216b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a020bc-066b-7f85-9447-11ab3aa854dc" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_01a020bc-066b-719d-b58f-0755001a216b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_01a020bc-066b-76d3-9777-65328b48653b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a020bc-066b-7f85-9447-11ab3aa854dc" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_01a020bc-066b-76d3-9777-65328b48653b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_01a020bc-066b-7fa8-aac9-ce4d8570f5eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a020bc-066b-7f85-9447-11ab3aa854dc" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_01a020bc-066b-7fa8-aac9-ce4d8570f5eb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosureAbstract_01a020bc-066b-7174-a477-12dce7b0ef99" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a020bc-066b-76df-9dd6-c8bf8ed82f0f" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_01a020bc-066b-7174-a477-12dce7b0ef99" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAbstract_01a020bc-066b-7717-a44b-950fc883eae0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_01a020bc-066b-7174-a477-12dce7b0ef99" xlink:to="loc_us-gaap_CashAndCashEquivalentsAbstract_01a020bc-066b-7717-a44b-950fc883eae0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_01a020bc-066b-7363-ba96-e766d929face" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsAbstract_01a020bc-066b-7717-a44b-950fc883eae0" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_01a020bc-066b-7363-ba96-e766d929face" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MarketableSecuritiesAbstract_01a020bc-066b-78f1-9021-bae2bbb69fea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MarketableSecuritiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_01a020bc-066b-7174-a477-12dce7b0ef99" xlink:to="loc_us-gaap_MarketableSecuritiesAbstract_01a020bc-066b-78f1-9021-bae2bbb69fea" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentsFairValueDisclosure_01a020bc-066b-7b57-a84f-e9f4d537d2d0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InvestmentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MarketableSecuritiesAbstract_01a020bc-066b-78f1-9021-bae2bbb69fea" xlink:to="loc_us-gaap_InvestmentsFairValueDisclosure_01a020bc-066b-7b57-a84f-e9f4d537d2d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_01a020bc-066b-78a9-8134-6c2cae612919" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_01a020bc-066b-7174-a477-12dce7b0ef99" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_01a020bc-066b-78a9-8134-6c2cae612919" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_01a020bc-066b-71a5-a56e-802f2981e668" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a020bc-066b-76df-9dd6-c8bf8ed82f0f" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_01a020bc-066b-71a5-a56e-802f2981e668" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFairValueDisclosureAbstract_01a020bc-066b-742a-a7f5-1fb00854abfa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_01a020bc-066b-71a5-a56e-802f2981e668" xlink:to="loc_us-gaap_DebtInstrumentFairValueDisclosureAbstract_01a020bc-066b-742a-a7f5-1fb00854abfa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeLiability_01a020bc-066b-750d-8bb1-6e16126de204" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentFairValueDisclosureAbstract_01a020bc-066b-742a-a7f5-1fb00854abfa" xlink:to="loc_us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeLiability_01a020bc-066b-750d-8bb1-6e16126de204" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_01a020bc-066b-7ec2-a295-81b3bfd0a0ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a020bc-066b-76df-9dd6-c8bf8ed82f0f" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_01a020bc-066b-7ec2-a295-81b3bfd0a0ad" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/FAIRVALUEMEASUREMENTSScheduleoftheFairValueusingLevel3SignificantUnobservableInputsDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#FAIRVALUEMEASUREMENTSScheduleoftheFairValueusingLevel3SignificantUnobservableInputsDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/FAIRVALUEMEASUREMENTSScheduleoftheFairValueusingLevel3SignificantUnobservableInputsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_01a020bc-066b-70ae-9d99-9daa2e0e57c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_01a020bc-066b-7c0d-be4f-c5be8885ce5b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_01a020bc-066b-70ae-9d99-9daa2e0e57c4" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_01a020bc-066b-7c0d-be4f-c5be8885ce5b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_01a020bc-066b-7ed9-909a-cdc5e55aaee7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTable_01a020bc-066b-7c0d-be4f-c5be8885ce5b" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_01a020bc-066b-7ed9-909a-cdc5e55aaee7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_01a020bc-066b-7ed9-909a-cdc5e55aaee7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_01a020bc-066b-7ed9-909a-cdc5e55aaee7" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_01a020bc-066b-7ed9-909a-cdc5e55aaee7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_01a020bc-066b-7cdc-a432-af300755e807" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_01a020bc-066b-7ed9-909a-cdc5e55aaee7" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_01a020bc-066b-7cdc-a432-af300755e807" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmbeddedDerivativeFinancialInstrumentsMember_01a020bc-066b-725e-a868-e6d9bc233e69" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmbeddedDerivativeFinancialInstrumentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_01a020bc-066b-7cdc-a432-af300755e807" xlink:to="loc_us-gaap_EmbeddedDerivativeFinancialInstrumentsMember_01a020bc-066b-725e-a868-e6d9bc233e69" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_01a020bc-066b-7f9a-bc3b-d74b5d070398" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_01a020bc-066b-70ae-9d99-9daa2e0e57c4" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_01a020bc-066b-7f9a-bc3b-d74b5d070398" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_01a020bc-066b-76b8-b028-ea8051dc91e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_01a020bc-066b-7f9a-bc3b-d74b5d070398" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_01a020bc-066b-76b8-b028-ea8051dc91e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_01a020bc-066b-73cf-aeb5-a9b4ac9b6fb9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_01a020bc-066b-7f9a-bc3b-d74b5d070398" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_01a020bc-066b-73cf-aeb5-a9b4ac9b6fb9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_01a020bc-066b-7c0c-8e3c-b0a595b86ff4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/DEBTAdditionalInformationDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#DEBTAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/DEBTAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-748b-90e7-9e1621cbf7b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-748b-90e7-9e1621cbf7b1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-76c6-b2a0-a2e511534b68" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-748b-90e7-9e1621cbf7b1" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-76c6-b2a0-a2e511534b68" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-76c6-b2a0-a2e511534b68_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-76c6-b2a0-a2e511534b68" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-76c6-b2a0-a2e511534b68_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7273-b3e2-10c46f04749b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-76c6-b2a0-a2e511534b68" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7273-b3e2-10c46f04749b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_TermLoanMember_01a020bc-0701-7f3d-b54d-b4c203780af3" xlink:href="abos-20260930.xsd#abos_TermLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7273-b3e2-10c46f04749b" xlink:to="loc_abos_TermLoanMember_01a020bc-0701-7f3d-b54d-b4c203780af3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_TermLoanFirstTrancheMember_01a020bc-0701-719a-ba10-831770bea0e9" xlink:href="abos-20260930.xsd#abos_TermLoanFirstTrancheMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7273-b3e2-10c46f04749b" xlink:to="loc_abos_TermLoanFirstTrancheMember_01a020bc-0701-719a-ba10-831770bea0e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_TermLoanSecondTrancheMember_01a020bc-0701-7ffe-a0d5-d1dca0f97d99" xlink:href="abos-20260930.xsd#abos_TermLoanSecondTrancheMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7273-b3e2-10c46f04749b" xlink:to="loc_abos_TermLoanSecondTrancheMember_01a020bc-0701-7ffe-a0d5-d1dca0f97d99" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7f49-ac66-b1afdbdab30c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-748b-90e7-9e1621cbf7b1" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7f49-ac66-b1afdbdab30c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-7f49-ac66-b1afdbdab30c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7f49-ac66-b1afdbdab30c" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-7f49-ac66-b1afdbdab30c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-7913-8144-dd7947c3b53c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7f49-ac66-b1afdbdab30c" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-7913-8144-dd7947c3b53c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_01a020bc-0701-73b1-85e3-2b408bb85a38" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-7913-8144-dd7947c3b53c" xlink:to="loc_us-gaap_SecuredDebtMember_01a020bc-0701-73b1-85e3-2b408bb85a38" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_01a020bc-0701-74be-a008-49ad45967d18" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-748b-90e7-9e1621cbf7b1" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_01a020bc-0701-74be-a008-49ad45967d18" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_01a020bc-0701-74be-a008-49ad45967d18_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_01a020bc-0701-74be-a008-49ad45967d18" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_01a020bc-0701-74be-a008-49ad45967d18_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_01a020bc-0701-728b-b116-3e91bc127b87" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_01a020bc-0701-74be-a008-49ad45967d18" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_01a020bc-0701-728b-b116-3e91bc127b87" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_01a020bc-0701-7139-8d5d-0b21f13cde54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a020bc-0701-728b-b116-3e91bc127b87" xlink:to="loc_us-gaap_WarrantMember_01a020bc-0701-7139-8d5d-0b21f13cde54" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_01a020bc-0701-7bfe-a4b3-0ce6424de8ba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-748b-90e7-9e1621cbf7b1" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_01a020bc-0701-7bfe-a4b3-0ce6424de8ba" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a020bc-0701-7bfe-a4b3-0ce6424de8ba_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_01a020bc-0701-7bfe-a4b3-0ce6424de8ba" xlink:to="loc_us-gaap_EquityComponentDomain_01a020bc-0701-7bfe-a4b3-0ce6424de8ba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a020bc-0701-759d-b997-f7aa3f58f613" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_01a020bc-0701-7bfe-a4b3-0ce6424de8ba" xlink:to="loc_us-gaap_EquityComponentDomain_01a020bc-0701-759d-b997-f7aa3f58f613" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_01a020bc-0701-7261-9af2-505102641691" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a020bc-0701-759d-b997-f7aa3f58f613" xlink:to="loc_us-gaap_CommonStockMember_01a020bc-0701-7261-9af2-505102641691" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_01a020bc-0701-7d5f-83b9-53c2790e4d3d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_01a020bc-0701-7d5f-83b9-53c2790e4d3d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_01a020bc-0701-7b09-bfac-0d0dea3207be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_01a020bc-0701-7b09-bfac-0d0dea3207be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_01a020bc-0701-79bd-a168-a2b64f83c489" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_01a020bc-0701-79bd-a168-a2b64f83c489" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFeeAmount_01a020bc-0701-7fd6-a8bb-7179dfa4f7d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFeeAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_us-gaap_DebtInstrumentFeeAmount_01a020bc-0701-7fd6-a8bb-7179dfa4f7d8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_DebtInstrumentMinimumRequiredPayment_01a020bc-0701-7159-8731-a1efc943852b" xlink:href="abos-20260930.xsd#abos_DebtInstrumentMinimumRequiredPayment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_abos_DebtInstrumentMinimumRequiredPayment_01a020bc-0701-7159-8731-a1efc943852b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_DebtInstrumentMinimumRequiredPaymentOutstandingDebtRequirement_01a020bc-0701-7f86-bae3-acb6d935644c" xlink:href="abos-20260930.xsd#abos_DebtInstrumentMinimumRequiredPaymentOutstandingDebtRequirement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_abos_DebtInstrumentMinimumRequiredPaymentOutstandingDebtRequirement_01a020bc-0701-7f86-bae3-acb6d935644c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_DebtInstrumentConvertibleIfConvertedValue_01a020bc-0701-78cf-b12f-ec7ce263ac3a" xlink:href="abos-20260930.xsd#abos_DebtInstrumentConvertibleIfConvertedValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_abos_DebtInstrumentConvertibleIfConvertedValue_01a020bc-0701-78cf-b12f-ec7ce263ac3a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_01a020bc-0701-7238-abac-3e7d3ffc20e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleConversionPrice1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_01a020bc-0701-7238-abac-3e7d3ffc20e9" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_DebtInstrumentCovenantProceedsFromIssuanceOfDebtRequired_01a020bc-0701-7e61-9cee-220404291f90" xlink:href="abos-20260930.xsd#abos_DebtInstrumentCovenantProceedsFromIssuanceOfDebtRequired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_abos_DebtInstrumentCovenantProceedsFromIssuanceOfDebtRequired_01a020bc-0701-7e61-9cee-220404291f90" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_01a020bc-0701-7311-92d0-1e26e6d4b23c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_01a020bc-0701-7311-92d0-1e26e6d4b23c" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstanding_01a020bc-0701-7027-a3e7-7cd106c494f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantsAndRightsOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_us-gaap_WarrantsAndRightsOutstanding_01a020bc-0701-7027-a3e7-7cd106c494f0" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_01a020bc-0701-71b4-97a7-25d4e0c1a390" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_01a020bc-0701-71b4-97a7-25d4e0c1a390" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeLiability_01a020bc-0701-78aa-b62d-46659815ccce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeLiability_01a020bc-0701-78aa-b62d-46659815ccce" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_01a020bc-0701-7ad7-968f-3d672ae0f26f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_01a020bc-0701-7ad7-968f-3d672ae0f26f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfLongTermDebt_01a12e0a-9eef-786a-8ec8-6a7cc714d7f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RepaymentsOfLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_us-gaap_RepaymentsOfLongTermDebt_01a12e0a-9eef-786a-8ec8-6a7cc714d7f8" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateDuringPeriod_01a020bc-0701-7419-8128-2b3d2c5c560f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-72d9-9f36-6bc7902062ed" xlink:to="loc_us-gaap_DebtInstrumentInterestRateDuringPeriod_01a020bc-0701-7419-8128-2b3d2c5c560f" xlink:type="arc" order="15"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/DEBTScheduleofOutstandingDebtDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#DEBTScheduleofOutstandingDebtDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/DEBTScheduleofOutstandingDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7070-bb48-2924efa2d7e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-720e-afb0-2114a3c2e3db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7070-bb48-2924efa2d7e7" xlink:to="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-720e-afb0-2114a3c2e3db" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-762d-9d27-b04a5b24fa18" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-720e-afb0-2114a3c2e3db" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-762d-9d27-b04a5b24fa18" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-762d-9d27-b04a5b24fa18_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-762d-9d27-b04a5b24fa18" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-762d-9d27-b04a5b24fa18_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7efa-8f88-fa9be3369de6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-762d-9d27-b04a5b24fa18" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7efa-8f88-fa9be3369de6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_TermLoanMember_01a020bc-0701-7472-b50a-5b72a98ede1e" xlink:href="abos-20260930.xsd#abos_TermLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7efa-8f88-fa9be3369de6" xlink:to="loc_abos_TermLoanMember_01a020bc-0701-7472-b50a-5b72a98ede1e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7b23-9d72-9b42ef3ebbf1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-720e-afb0-2114a3c2e3db" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7b23-9d72-9b42ef3ebbf1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-7b23-9d72-9b42ef3ebbf1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7b23-9d72-9b42ef3ebbf1" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-7b23-9d72-9b42ef3ebbf1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-721d-8ff8-bb079f9c0c33" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7b23-9d72-9b42ef3ebbf1" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-721d-8ff8-bb079f9c0c33" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_01a020bc-0701-7c53-9f13-904eb58b09cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-721d-8ff8-bb079f9c0c33" xlink:to="loc_us-gaap_SecuredDebtMember_01a020bc-0701-7c53-9f13-904eb58b09cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_01a020bc-0701-7b4d-99ca-19ae8dbefdf4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7070-bb48-2924efa2d7e7" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_01a020bc-0701-7b4d-99ca-19ae8dbefdf4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeLiability_01a020bc-0701-7d6b-8372-4f9280e52ebe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7070-bb48-2924efa2d7e7" xlink:to="loc_us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeLiability_01a020bc-0701-7d6b-8372-4f9280e52ebe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_01a020bc-0701-72d6-919b-8a2008085d90" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7070-bb48-2924efa2d7e7" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_01a020bc-0701-72d6-919b-8a2008085d90" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtLongtermAndShorttermCombinedAmount_01a020bc-0701-7cee-8fb8-ad5f219375fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtLongtermAndShorttermCombinedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7070-bb48-2924efa2d7e7" xlink:to="loc_us-gaap_DebtLongtermAndShorttermCombinedAmount_01a020bc-0701-7cee-8fb8-ad5f219375fd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermBorrowings_01a020bc-0701-71f9-9e76-b653c8b0199f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermBorrowings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7070-bb48-2924efa2d7e7" xlink:to="loc_us-gaap_ShortTermBorrowings_01a020bc-0701-71f9-9e76-b653c8b0199f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_01a020bc-0701-7d9b-926b-602056b1d940" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7070-bb48-2924efa2d7e7" xlink:to="loc_us-gaap_LongTermDebt_01a020bc-0701-7d9b-926b-602056b1d940" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFeeAmount_01a020bc-0701-7e94-aac8-46f5855f91ef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFeeAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7070-bb48-2924efa2d7e7" xlink:to="loc_us-gaap_DebtInstrumentFeeAmount_01a020bc-0701-7e94-aac8-46f5855f91ef" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/DEBTScheduleofInterestExpenseDebtDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#DEBTScheduleofInterestExpenseDebtDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/DEBTScheduleofInterestExpenseDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7812-bb80-eb46a6786e6c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-701d-b656-57bef1cf6e22" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7812-bb80-eb46a6786e6c" xlink:to="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-701d-b656-57bef1cf6e22" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-7511-b5f0-a6aac878fc47" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-701d-b656-57bef1cf6e22" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-7511-b5f0-a6aac878fc47" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7511-b5f0-a6aac878fc47_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-7511-b5f0-a6aac878fc47" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7511-b5f0-a6aac878fc47_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7af1-8b05-248075efc532" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-7511-b5f0-a6aac878fc47" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7af1-8b05-248075efc532" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_TermLoanMember_01a020bc-0701-7210-956c-2048e554a334" xlink:href="abos-20260930.xsd#abos_TermLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7af1-8b05-248075efc532" xlink:to="loc_abos_TermLoanMember_01a020bc-0701-7210-956c-2048e554a334" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7517-9e67-69fe6b1a1801" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-701d-b656-57bef1cf6e22" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7517-9e67-69fe6b1a1801" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-7517-9e67-69fe6b1a1801_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7517-9e67-69fe6b1a1801" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-7517-9e67-69fe6b1a1801_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-7117-b362-2687cfbe77ee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7517-9e67-69fe6b1a1801" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-7117-b362-2687cfbe77ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_01a020bc-0701-779c-b1f6-ec724a57dd3c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-7117-b362-2687cfbe77ee" xlink:to="loc_us-gaap_SecuredDebtMember_01a020bc-0701-779c-b1f6-ec724a57dd3c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebtExcludingAmortization_01a020bc-0701-7d83-9085-34dacb55e5f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDebtExcludingAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7812-bb80-eb46a6786e6c" xlink:to="loc_us-gaap_InterestExpenseDebtExcludingAmortization_01a020bc-0701-7d83-9085-34dacb55e5f8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfDebtDiscountPremium_01a020bc-0701-7d06-9315-9f672830ede5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AmortizationOfDebtDiscountPremium"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7812-bb80-eb46a6786e6c" xlink:to="loc_us-gaap_AmortizationOfDebtDiscountPremium_01a020bc-0701-7d06-9315-9f672830ede5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_01a020bc-0701-7278-add1-2e04e0c0e1e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7812-bb80-eb46a6786e6c" xlink:to="loc_us-gaap_InterestExpenseDebt_01a020bc-0701-7278-add1-2e04e0c0e1e7" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/DEBTScheduleofAggregatePrincipalPaymentsDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#DEBTScheduleofAggregatePrincipalPaymentsDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/DEBTScheduleofAggregatePrincipalPaymentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7ec2-a3a9-cac4b1280ce2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-7943-9cce-84a3fcb2e3ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7ec2-a3a9-cac4b1280ce2" xlink:to="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-7943-9cce-84a3fcb2e3ea" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-7e02-90ee-00b5c0cc4f54" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-7943-9cce-84a3fcb2e3ea" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-7e02-90ee-00b5c0cc4f54" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7e02-90ee-00b5c0cc4f54_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-7e02-90ee-00b5c0cc4f54" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7e02-90ee-00b5c0cc4f54_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7462-ae7f-89784074a409" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a020bc-0701-7e02-90ee-00b5c0cc4f54" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7462-ae7f-89784074a409" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_TermLoanMember_01a020bc-0701-7e54-874b-49ac4a4ed441" xlink:href="abos-20260930.xsd#abos_TermLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a020bc-0701-7462-ae7f-89784074a409" xlink:to="loc_abos_TermLoanMember_01a020bc-0701-7e54-874b-49ac4a4ed441" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7baa-8369-b04af60ee638" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a020bc-0701-7943-9cce-84a3fcb2e3ea" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7baa-8369-b04af60ee638" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-7baa-8369-b04af60ee638_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7baa-8369-b04af60ee638" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-7baa-8369-b04af60ee638_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-75a7-9172-1f1f3e6021cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a020bc-0701-7baa-8369-b04af60ee638" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-75a7-9172-1f1f3e6021cb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_01a020bc-0701-7737-a99d-8fd386e05170" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a020bc-0701-75a7-9172-1f1f3e6021cb" xlink:to="loc_us-gaap_SecuredDebtMember_01a020bc-0701-7737-a99d-8fd386e05170" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear_01a020bc-0701-7b13-92e1-29fa44bd1e6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7ec2-a3a9-cac4b1280ce2" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear_01a020bc-0701-7b13-92e1-29fa44bd1e6f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths_01a020bc-0701-7bc2-8fcf-5092bfa5f16e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7ec2-a3a9-cac4b1280ce2" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths_01a020bc-0701-7bc2-8fcf-5092bfa5f16e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_01a020bc-0701-7c32-a494-977494b1797b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a020bc-0701-7ec2-a3a9-cac4b1280ce2" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_01a020bc-0701-7c32-a494-977494b1797b" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/STOCKHOLDERSEQUITYDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#STOCKHOLDERSEQUITYDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/STOCKHOLDERSEQUITYDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_01a020bc-0701-7a84-b694-e7704a95a324" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_01a020bc-0701-7a84-b694-e7704a95a324" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_01a020bc-0701-7e1f-b2cd-344d280f3094" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_01a020bc-0701-7a84-b694-e7704a95a324" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_01a020bc-0701-7e1f-b2cd-344d280f3094" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a020bc-0701-7e1f-b2cd-344d280f3094_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_01a020bc-0701-7e1f-b2cd-344d280f3094" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a020bc-0701-7e1f-b2cd-344d280f3094_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a020bc-0701-7b4e-8485-83004449c8c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_01a020bc-0701-7e1f-b2cd-344d280f3094" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a020bc-0701-7b4e-8485-83004449c8c2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivatePlacementMember_01a020bc-0701-7a3a-a5c4-68de7f1e1f9b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a020bc-0701-7b4e-8485-83004449c8c2" xlink:to="loc_us-gaap_PrivatePlacementMember_01a020bc-0701-7a3a-a5c4-68de7f1e1f9b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_RegistrationStatementMember_01a020bc-0701-7869-a61c-e0db557e9f8a" xlink:href="abos-20260930.xsd#abos_RegistrationStatementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a020bc-0701-7b4e-8485-83004449c8c2" xlink:to="loc_abos_RegistrationStatementMember_01a020bc-0701-7869-a61c-e0db557e9f8a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_AtTheMarketOfferingMember_01a020bc-0701-77f7-a442-18fed8fcd39d" xlink:href="abos-20260930.xsd#abos_AtTheMarketOfferingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a020bc-0701-7b4e-8485-83004449c8c2" xlink:to="loc_abos_AtTheMarketOfferingMember_01a020bc-0701-77f7-a442-18fed8fcd39d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_A2024RegistrationStatementMember_01a020bc-0701-74e3-9adb-c57dcbf577a0" xlink:href="abos-20260930.xsd#abos_A2024RegistrationStatementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_01a020bc-0701-7b4e-8485-83004449c8c2" xlink:to="loc_abos_A2024RegistrationStatementMember_01a020bc-0701-74e3-9adb-c57dcbf577a0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_01a020bc-0701-7efe-a5f1-46b6c482a824" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_01a020bc-0701-7a84-b694-e7704a95a324" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_01a020bc-0701-7efe-a5f1-46b6c482a824" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a020bc-0701-7efe-a5f1-46b6c482a824_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_01a020bc-0701-7efe-a5f1-46b6c482a824" xlink:to="loc_us-gaap_EquityComponentDomain_01a020bc-0701-7efe-a5f1-46b6c482a824_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a020bc-0701-7711-ba1a-13d75ce5beb8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_01a020bc-0701-7efe-a5f1-46b6c482a824" xlink:to="loc_us-gaap_EquityComponentDomain_01a020bc-0701-7711-ba1a-13d75ce5beb8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_01a020bc-0701-790a-843f-4f73bc6eeb41" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a020bc-0701-7711-ba1a-13d75ce5beb8" xlink:to="loc_us-gaap_CommonStockMember_01a020bc-0701-790a-843f-4f73bc6eeb41" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_01a020bc-0701-7ad8-97cf-b3de2305eb1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_01a020bc-0701-7a84-b694-e7704a95a324" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_01a020bc-0701-7ad8-97cf-b3de2305eb1d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_01a020bc-0701-7ad8-97cf-b3de2305eb1d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_01a020bc-0701-7ad8-97cf-b3de2305eb1d" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_01a020bc-0701-7ad8-97cf-b3de2305eb1d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_01a020bc-0701-7030-9db5-fd97b854924e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_01a020bc-0701-7ad8-97cf-b3de2305eb1d" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_01a020bc-0701-7030-9db5-fd97b854924e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_01a020bc-0701-78da-abb3-c38f0073aaaa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a020bc-0701-7030-9db5-fd97b854924e" xlink:to="loc_us-gaap_WarrantMember_01a020bc-0701-78da-abb3-c38f0073aaaa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_CommonStockAndPreferredStockSharesAuthorized_01a020bc-0701-721e-b152-f1728d0bc742" xlink:href="abos-20260930.xsd#abos_CommonStockAndPreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_abos_CommonStockAndPreferredStockSharesAuthorized_01a020bc-0701-721e-b152-f1728d0bc742" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_01a020bc-0701-715b-a7c0-dae8f230e433" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_01a020bc-0701-715b-a7c0-dae8f230e433" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_01a020bc-0701-7f5a-9f02-232963bf7e35" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_01a020bc-0701-7f5a-9f02-232963bf7e35" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_01a020bc-0701-71e7-8aeb-a2272579a618" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_01a020bc-0701-71e7-8aeb-a2272579a618" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_01a020bc-0701-71fd-b872-8ce3de391183" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_01a020bc-0701-71fd-b872-8ce3de391183" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_CommonStockNumberOfVotingRights_01a020bc-0701-7be1-a2d9-455bd650544b" xlink:href="abos-20260930.xsd#abos_CommonStockNumberOfVotingRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_abos_CommonStockNumberOfVotingRights_01a020bc-0701-7be1-a2d9-455bd650544b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_01a020bc-0701-71e0-85f3-9588ab1c89a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_01a020bc-0701-71e0-85f3-9588ab1c89a4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_01a020bc-0701-78b4-9b5e-5a57696597ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_01a020bc-0701-78b4-9b5e-5a57696597ec" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_01a020bc-0701-7e9d-bc24-ca98fa9ddd84" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfPrivatePlacement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfPrivatePlacement_01a020bc-0701-7e9d-bc24-ca98fa9ddd84" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfStockIssuanceCosts_01a020bc-0701-7f79-beaf-3aad0a911444" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsOfStockIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_PaymentsOfStockIssuanceCosts_01a020bc-0701-7f79-beaf-3aad0a911444" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_SaleOfStockMaximumAuthorizedAmount_01a020bc-0701-7134-b159-9bbbcb7633e3" xlink:href="abos-20260930.xsd#abos_SaleOfStockMaximumAuthorizedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_abos_SaleOfStockMaximumAuthorizedAmount_01a020bc-0701-7134-b159-9bbbcb7633e3" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_SaleOfStockCommissionRateToSalesAgents_01a020bc-0701-7ac0-88f5-b55780fface4" xlink:href="abos-20260930.xsd#abos_SaleOfStockCommissionRateToSalesAgents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_abos_SaleOfStockCommissionRateToSalesAgents_01a020bc-0701-7ac0-88f5-b55780fface4" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_01a16512-9d7b-7462-a4f7-884923c782f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_01a16512-9d7b-7462-a4f7-884923c782f5" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockSharesIssuedNetOfSharesForTaxWithholdings_01a020bc-0701-7ba4-beae-d1757b932b33" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockSharesIssuedNetOfSharesForTaxWithholdings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_RestrictedStockSharesIssuedNetOfSharesForTaxWithholdings_01a020bc-0701-7ba4-beae-d1757b932b33" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_01a020bc-0701-790e-99c4-f23eac28b577" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_01a020bc-0701-790e-99c4-f23eac28b577" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_01a020bc-0701-7c2b-a233-9d733ca37bb8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_01a020bc-0701-7c2b-a233-9d733ca37bb8" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingTerm_01a020bc-0701-76ff-8ecb-873cc770beac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantsAndRightsOutstandingTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingTerm_01a020bc-0701-76ff-8ecb-873cc770beac" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstanding_01a020bc-0701-7a5a-95fe-20e05fd8666b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantsAndRightsOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a020bc-0701-7013-ad7e-22a42113281f" xlink:to="loc_us-gaap_WarrantsAndRightsOutstanding_01a020bc-0701-7a5a-95fe-20e05fd8666b" xlink:type="arc" order="17"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/STOCKBASEDCOMPENSATIONAdditionalInformationDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#STOCKBASEDCOMPENSATIONAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/STOCKBASEDCOMPENSATIONAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a020bc-0701-7d15-93d0-80bb9aeb1046" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a020bc-0701-7d15-93d0-80bb9aeb1046" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_01a020bc-0701-7473-a09f-1290e5b37d49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a020bc-0701-7d15-93d0-80bb9aeb1046" xlink:to="loc_us-gaap_PlanNameAxis_01a020bc-0701-7473-a09f-1290e5b37d49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_01a020bc-0701-7473-a09f-1290e5b37d49_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_01a020bc-0701-7473-a09f-1290e5b37d49" xlink:to="loc_us-gaap_PlanNameDomain_01a020bc-0701-7473-a09f-1290e5b37d49_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_01a020bc-0701-77af-9711-93ee3cf327aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_01a020bc-0701-7473-a09f-1290e5b37d49" xlink:to="loc_us-gaap_PlanNameDomain_01a020bc-0701-77af-9711-93ee3cf327aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_A2013EquityIncentivePlanMember_01a020bc-0701-7f74-87fa-ec8695a51154" xlink:href="abos-20260930.xsd#abos_A2013EquityIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_01a020bc-0701-77af-9711-93ee3cf327aa" xlink:to="loc_abos_A2013EquityIncentivePlanMember_01a020bc-0701-7f74-87fa-ec8695a51154" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_A2021EquityIncentivePlanMember_01a020bc-0701-70ea-9bfc-a2c836865d3b" xlink:href="abos-20260930.xsd#abos_A2021EquityIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_01a020bc-0701-77af-9711-93ee3cf327aa" xlink:to="loc_abos_A2021EquityIncentivePlanMember_01a020bc-0701-70ea-9bfc-a2c836865d3b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_01a020bc-0701-7fdf-ad74-1ac9a2ccbaa4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a020bc-0701-7d15-93d0-80bb9aeb1046" xlink:to="loc_us-gaap_StatementClassOfStockAxis_01a020bc-0701-7fdf-ad74-1ac9a2ccbaa4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_01a020bc-0701-7fdf-ad74-1ac9a2ccbaa4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_01a020bc-0701-7fdf-ad74-1ac9a2ccbaa4" xlink:to="loc_us-gaap_ClassOfStockDomain_01a020bc-0701-7fdf-ad74-1ac9a2ccbaa4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_01a020bc-0701-7400-8067-0fca3d356ab5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_01a020bc-0701-7fdf-ad74-1ac9a2ccbaa4" xlink:to="loc_us-gaap_ClassOfStockDomain_01a020bc-0701-7400-8067-0fca3d356ab5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_01a020bc-0701-7cfd-9da1-b776d9551ab1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_01a020bc-0701-7400-8067-0fca3d356ab5" xlink:to="loc_us-gaap_CommonStockMember_01a020bc-0701-7cfd-9da1-b776d9551ab1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_01a020bc-0701-7cd6-9b86-4dd614014af5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a020bc-0701-7d15-93d0-80bb9aeb1046" xlink:to="loc_us-gaap_AwardTypeAxis_01a020bc-0701-7cd6-9b86-4dd614014af5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0701-7cd6-9b86-4dd614014af5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_01a020bc-0701-7cd6-9b86-4dd614014af5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0701-7cd6-9b86-4dd614014af5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0701-79a3-850f-855315b08569" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_01a020bc-0701-7cd6-9b86-4dd614014af5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0701-79a3-850f-855315b08569" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_01a020bc-0701-7105-8c8e-6b5b87708aaa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0701-79a3-850f-855315b08569" xlink:to="loc_us-gaap_EmployeeStockOptionMember_01a020bc-0701-7105-8c8e-6b5b87708aaa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_01a020bc-0701-7fdd-81d6-3610942e1557" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0701-79a3-850f-855315b08569" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_01a020bc-0701-7fdd-81d6-3610942e1557" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_01a020bc-0701-7b88-9154-677c5a79c2be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0701-79a3-850f-855315b08569" xlink:to="loc_us-gaap_EmployeeStockMember_01a020bc-0701-7b88-9154-677c5a79c2be" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_01a020bc-0701-7712-8f00-5554adacc402" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a020bc-0701-7d15-93d0-80bb9aeb1046" xlink:to="loc_srt_RangeAxis_01a020bc-0701-7712-8f00-5554adacc402" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a020bc-0701-7712-8f00-5554adacc402_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_01a020bc-0701-7712-8f00-5554adacc402" xlink:to="loc_srt_RangeMember_01a020bc-0701-7712-8f00-5554adacc402_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a020bc-0701-78f0-ad46-a4c6529a800b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_01a020bc-0701-7712-8f00-5554adacc402" xlink:to="loc_srt_RangeMember_01a020bc-0701-78f0-ad46-a4c6529a800b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_01a020bc-0701-7c8f-b61c-b1ae73641f2b" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a020bc-0701-78f0-ad46-a4c6529a800b" xlink:to="loc_srt_MinimumMember_01a020bc-0701-7c8f-b61c-b1ae73641f2b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_01a020bc-0701-7642-b8d6-70628cd6db75" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a020bc-0701-78f0-ad46-a4c6529a800b" xlink:to="loc_srt_MaximumMember_01a020bc-0701-7642-b8d6-70628cd6db75" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateAxis_01a020bc-0701-71d7-aca6-c618a656208e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a020bc-0701-7d15-93d0-80bb9aeb1046" xlink:to="loc_us-gaap_AwardDateAxis_01a020bc-0701-71d7-aca6-c618a656208e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_01a020bc-0701-71d7-aca6-c618a656208e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardDateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardDateAxis_01a020bc-0701-71d7-aca6-c618a656208e" xlink:to="loc_us-gaap_AwardDateDomain_01a020bc-0701-71d7-aca6-c618a656208e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardDateDomain_01a020bc-0701-79ba-b491-75a09e1ed384" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardDateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardDateAxis_01a020bc-0701-71d7-aca6-c618a656208e" xlink:to="loc_us-gaap_AwardDateDomain_01a020bc-0701-79ba-b491-75a09e1ed384" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_AwardDateTwoMember_01a020bc-0701-7041-8879-3bc61a65e3a5" xlink:href="abos-20260930.xsd#abos_AwardDateTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_01a020bc-0701-79ba-b491-75a09e1ed384" xlink:to="loc_abos_AwardDateTwoMember_01a020bc-0701-7041-8879-3bc61a65e3a5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_AwardDateThreeMember_01a020bc-0701-7ab0-a0ef-96b6b6fad3c8" xlink:href="abos-20260930.xsd#abos_AwardDateThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_01a020bc-0701-79ba-b491-75a09e1ed384" xlink:to="loc_abos_AwardDateThreeMember_01a020bc-0701-7ab0-a0ef-96b6b6fad3c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_AwardDateOneMember_01a020bc-0701-797c-a930-83751445b101" xlink:href="abos-20260930.xsd#abos_AwardDateOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AwardDateDomain_01a020bc-0701-79ba-b491-75a09e1ed384" xlink:to="loc_abos_AwardDateOneMember_01a020bc-0701-797c-a930-83751445b101" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_01a020bc-0701-7f06-a0a0-443ec99c79b0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a020bc-0701-7d15-93d0-80bb9aeb1046" xlink:to="loc_us-gaap_VestingAxis_01a020bc-0701-7f06-a0a0-443ec99c79b0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_01a020bc-0701-7f06-a0a0-443ec99c79b0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_01a020bc-0701-7f06-a0a0-443ec99c79b0" xlink:to="loc_us-gaap_VestingDomain_01a020bc-0701-7f06-a0a0-443ec99c79b0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_01a020bc-0701-72e4-8a7f-fe8d7bb65e8f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_01a020bc-0701-7f06-a0a0-443ec99c79b0" xlink:to="loc_us-gaap_VestingDomain_01a020bc-0701-72e4-8a7f-fe8d7bb65e8f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_01a020bc-0701-7a34-8c7f-d63dd8ff42ee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_01a020bc-0701-72e4-8a7f-fe8d7bb65e8f" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_01a020bc-0701-7a34-8c7f-d63dd8ff42ee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_01a020bc-0701-74ee-be59-3ff69bd6e7d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_01a020bc-0701-72e4-8a7f-fe8d7bb65e8f" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_01a020bc-0701-74ee-be59-3ff69bd6e7d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheThreeMember_01a020bc-0701-73da-be2c-9e16d9b06556" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationAwardTrancheThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_01a020bc-0701-72e4-8a7f-fe8d7bb65e8f" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheThreeMember_01a020bc-0701-73da-be2c-9e16d9b06556" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_01a020bc-0701-7150-b4ae-32c2dfadc5db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_01a020bc-0701-7150-b4ae-32c2dfadc5db" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_01a020bc-0701-7801-9624-7fb493311c09" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_01a020bc-0701-7801-9624-7fb493311c09" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum_01a020bc-0701-7ff9-8fd8-0c9ff72cf8e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum_01a020bc-0701-7ff9-8fd8-0c9ff72cf8e0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_01a020bc-0701-7d85-a43e-c1f6e14eefeb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_01a020bc-0701-7d85-a43e-c1f6e14eefeb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_ShareBasedPaymentArrangementMaximumThresholdCommonStockIssuableOnExerciseOfIncentiveStockOptions_01a020bc-0701-7b70-bf82-caedb91420c5" xlink:href="abos-20260930.xsd#abos_ShareBasedPaymentArrangementMaximumThresholdCommonStockIssuableOnExerciseOfIncentiveStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_abos_ShareBasedPaymentArrangementMaximumThresholdCommonStockIssuableOnExerciseOfIncentiveStockOptions_01a020bc-0701-7b70-bf82-caedb91420c5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_01a020bc-0701-78b1-b1fa-1bf5c1a1b368" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_01a020bc-0701-78b1-b1fa-1bf5c1a1b368" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_01a020bc-0701-7e4e-802b-226e8581f8e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_01a020bc-0701-7e4e-802b-226e8581f8e7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_01a020bc-0701-7864-af6b-838cbe52a02c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_01a020bc-0701-7864-af6b-838cbe52a02c" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_01a020bc-0701-7f47-96b5-73ab096dc5ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_01a020bc-0701-7f47-96b5-73ab096dc5ea" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_ShareBasedCompensationArrangementByShareBasedPaymentAwardCliffPeriod_01a020bc-0701-741f-a6f2-6509e20393f1" xlink:href="abos-20260930.xsd#abos_ShareBasedCompensationArrangementByShareBasedPaymentAwardCliffPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_abos_ShareBasedCompensationArrangementByShareBasedPaymentAwardCliffPeriod_01a020bc-0701-741f-a6f2-6509e20393f1" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_01a020bc-0701-78f2-8aba-d18f2be45e79" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_01a020bc-0701-78f2-8aba-d18f2be45e79" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_01a020bc-0701-72e9-bbcb-5f0651f92aa8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_01a020bc-0701-72e9-bbcb-5f0651f92aa8" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_01a020bc-0701-70a5-8480-8144d45fe5c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_01a020bc-0701-70a5-8480-8144d45fe5c4" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_01a020bc-0701-7498-99ee-0b1fd67709c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_01a020bc-0701-7498-99ee-0b1fd67709c9" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_01a020bc-0701-76d1-8b2b-910aa5aa2bfe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_01a020bc-0701-76d1-8b2b-910aa5aa2bfe" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_01a020bc-0701-73ed-9133-9d7eebe2b58f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_01a020bc-0701-73ed-9133-9d7eebe2b58f" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_01a020bc-0701-74c3-b115-190e9b8a5a03" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_01a020bc-0701-74c3-b115-190e9b8a5a03" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesOfCommonStockOutstandingMaximum_01a020bc-0701-79cf-bcf6-3dc0315e5548" xlink:href="abos-20260930.xsd#abos_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesOfCommonStockOutstandingMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_abos_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesOfCommonStockOutstandingMaximum_01a020bc-0701-79cf-bcf6-3dc0315e5548" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_01a020bc-0701-7e08-9f5b-f6cf43dc66d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0701-7e6c-8188-4fe61fc3ea3d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_01a020bc-0701-7e08-9f5b-f6cf43dc66d2" xlink:type="arc" order="18"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/STOCKBASEDCOMPENSATIONScheduleofShareBasedCompensationExpenseRelatedtoStockOptionsDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#STOCKBASEDCOMPENSATIONScheduleofShareBasedCompensationExpenseRelatedtoStockOptionsDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/STOCKBASEDCOMPENSATIONScheduleofShareBasedCompensationExpenseRelatedtoStockOptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a020bc-0701-76f6-89f9-73837a674f23" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a020bc-0701-73dc-9c42-9212f8462f4e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a020bc-0701-76f6-89f9-73837a674f23" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a020bc-0701-73dc-9c42-9212f8462f4e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_01a020bc-0701-7da8-9338-2727e9bd47d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a020bc-0701-73dc-9c42-9212f8462f4e" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_01a020bc-0701-7da8-9338-2727e9bd47d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a020bc-0701-75d2-b3a4-61a3fd8191b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a020bc-0701-76f6-89f9-73837a674f23" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a020bc-0701-75d2-b3a4-61a3fd8191b1" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/STOCKBASEDCOMPENSATIONScheduleofShareBasedCompensationExpenseRelatedtoStockOptionsDetails_1" xlink:type="simple" xlink:href="abos-20260930.xsd#STOCKBASEDCOMPENSATIONScheduleofShareBasedCompensationExpenseRelatedtoStockOptionsDetails_1"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/STOCKBASEDCOMPENSATIONScheduleofShareBasedCompensationExpenseRelatedtoStockOptionsDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a020bc-0701-76f6-89f9-73837a674f23" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a020bc-0701-73dc-9c42-9212f8462f4e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a020bc-0701-76f6-89f9-73837a674f23" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a020bc-0701-73dc-9c42-9212f8462f4e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a020bc-0701-75d2-b3a4-61a3fd8191b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a020bc-0701-76f6-89f9-73837a674f23" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a020bc-0701-75d2-b3a4-61a3fd8191b1" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/STOCKBASEDCOMPENSATIONScheduleofAssumptionsUsedforValuationofFairValueofStockOptionsDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#STOCKBASEDCOMPENSATIONScheduleofAssumptionsUsedforValuationofFairValueofStockOptionsDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/STOCKBASEDCOMPENSATIONScheduleofAssumptionsUsedforValuationofFairValueofStockOptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a020bc-0701-7998-8d48-33d8ff858791" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a020bc-0701-7a2b-8424-fad3fd0fab0d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a020bc-0701-7998-8d48-33d8ff858791" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a020bc-0701-7a2b-8424-fad3fd0fab0d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_01a020bc-0701-7c7a-a0cd-272e3445ffa9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a020bc-0701-7a2b-8424-fad3fd0fab0d" xlink:to="loc_us-gaap_AwardTypeAxis_01a020bc-0701-7c7a-a0cd-272e3445ffa9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0701-7c7a-a0cd-272e3445ffa9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_01a020bc-0701-7c7a-a0cd-272e3445ffa9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0701-7c7a-a0cd-272e3445ffa9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0701-7016-9f1d-ac5b768b792c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_01a020bc-0701-7c7a-a0cd-272e3445ffa9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0701-7016-9f1d-ac5b768b792c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_01a020bc-0701-79b5-a11d-b33e02fc1a20" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0701-7016-9f1d-ac5b768b792c" xlink:to="loc_us-gaap_EmployeeStockOptionMember_01a020bc-0701-79b5-a11d-b33e02fc1a20" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_01a020bc-0701-7a0f-9ccd-0f4c44d53cb9" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_01a020bc-0701-7a2b-8424-fad3fd0fab0d" xlink:to="loc_srt_RangeAxis_01a020bc-0701-7a0f-9ccd-0f4c44d53cb9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a020bc-0701-7a0f-9ccd-0f4c44d53cb9_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_01a020bc-0701-7a0f-9ccd-0f4c44d53cb9" xlink:to="loc_srt_RangeMember_01a020bc-0701-7a0f-9ccd-0f4c44d53cb9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a020bc-0701-7c34-b4cf-add5eaf9ade5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_01a020bc-0701-7a0f-9ccd-0f4c44d53cb9" xlink:to="loc_srt_RangeMember_01a020bc-0701-7c34-b4cf-add5eaf9ade5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_01a020bc-0701-745c-b247-21db56ac146c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a020bc-0701-7c34-b4cf-add5eaf9ade5" xlink:to="loc_srt_MinimumMember_01a020bc-0701-745c-b247-21db56ac146c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_01a020bc-0701-7c9c-9faa-b05c67fc40cb" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a020bc-0701-7c34-b4cf-add5eaf9ade5" xlink:to="loc_srt_MaximumMember_01a020bc-0701-7c9c-9faa-b05c67fc40cb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum_01a020bc-0701-7e94-8224-e47f9bcc7c35" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a020bc-0701-7998-8d48-33d8ff858791" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum_01a020bc-0701-7e94-8224-e47f9bcc7c35" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum_01a020bc-0701-76f6-a7af-28b9d1d9c039" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a020bc-0701-7998-8d48-33d8ff858791" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum_01a020bc-0701-76f6-a7af-28b9d1d9c039" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_01a020bc-0701-7b8e-9090-e75a797a1814" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a020bc-0701-7998-8d48-33d8ff858791" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_01a020bc-0701-7b8e-9090-e75a797a1814" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum_01a020bc-0701-736b-b6de-7b3225501a06" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a020bc-0701-7998-8d48-33d8ff858791" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum_01a020bc-0701-736b-b6de-7b3225501a06" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum_01a020bc-0701-7b0f-b003-e31317802ab5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a020bc-0701-7998-8d48-33d8ff858791" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum_01a020bc-0701-7b0f-b003-e31317802ab5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_01a020bc-0701-723e-9473-6c8d57e5afb4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_01a020bc-0701-7998-8d48-33d8ff858791" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_01a020bc-0701-723e-9473-6c8d57e5afb4" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/STOCKBASEDCOMPENSATIONScheduleofNonvestedRestrictedStockUnitsActivityDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#STOCKBASEDCOMPENSATIONScheduleofNonvestedRestrictedStockUnitsActivityDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/STOCKBASEDCOMPENSATIONScheduleofNonvestedRestrictedStockUnitsActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0702-7fd7-81f8-22e72091e857" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a020bc-0702-74b7-88e0-df1cf2c416ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0702-7fd7-81f8-22e72091e857" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a020bc-0702-74b7-88e0-df1cf2c416ff" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_01a020bc-0702-7951-9977-5616b77b4b4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a020bc-0702-74b7-88e0-df1cf2c416ff" xlink:to="loc_us-gaap_AwardTypeAxis_01a020bc-0702-7951-9977-5616b77b4b4a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0702-7951-9977-5616b77b4b4a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_01a020bc-0702-7951-9977-5616b77b4b4a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0702-7951-9977-5616b77b4b4a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0702-76dc-992c-0bcc1c3f3df0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_01a020bc-0702-7951-9977-5616b77b4b4a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0702-76dc-992c-0bcc1c3f3df0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_01a020bc-0702-7f61-9f15-d3f8171ab977" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a020bc-0702-76dc-992c-0bcc1c3f3df0" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_01a020bc-0702-7f61-9f15-d3f8171ab977" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_01a020bc-0702-7ac0-8e3c-ef9c6723be7f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0702-7fd7-81f8-22e72091e857" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_01a020bc-0702-7ac0-8e3c-ef9c6723be7f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_01a020bc-0702-71b5-b361-be6ff31cf83e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_01a020bc-0702-7ac0-8e3c-ef9c6723be7f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_01a020bc-0702-71b5-b361-be6ff31cf83e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_01a020bc-0702-79dc-a23b-60e8b8f9202e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_01a020bc-0702-7ac0-8e3c-ef9c6723be7f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_01a020bc-0702-79dc-a23b-60e8b8f9202e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_01a020bc-0702-7136-867b-86bf6addd40d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_01a020bc-0702-7ac0-8e3c-ef9c6723be7f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_01a020bc-0702-7136-867b-86bf6addd40d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_01a020bc-0702-7223-92c1-ed769d491e5b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_01a020bc-0702-7ac0-8e3c-ef9c6723be7f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_01a020bc-0702-7223-92c1-ed769d491e5b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_01a020bc-0702-78de-b5cc-b8de479e0302" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_01a020bc-0702-7187-8ff0-dbc54f3efb8d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a020bc-0702-7fd7-81f8-22e72091e857" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_01a020bc-0702-7187-8ff0-dbc54f3efb8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_01a020bc-0702-740e-97f5-b138f663d7d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_01a020bc-0702-7187-8ff0-dbc54f3efb8d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_01a020bc-0702-740e-97f5-b138f663d7d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_01a020bc-0702-7f23-8107-d674917fa652" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_01a020bc-0702-7187-8ff0-dbc54f3efb8d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_01a020bc-0702-7f23-8107-d674917fa652" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_01a020bc-0702-7d22-be71-9c8d0a88bb9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_01a020bc-0702-7187-8ff0-dbc54f3efb8d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_01a020bc-0702-7d22-be71-9c8d0a88bb9c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_01a020bc-0702-72a6-b6fd-8f51b8c83ec5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_01a020bc-0702-7187-8ff0-dbc54f3efb8d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_01a020bc-0702-72a6-b6fd-8f51b8c83ec5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_01a020bc-0702-7040-97d4-9ba4751220c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/SEGMENTREPORTINGScheduleofFinancialInformationaboutCompanysOperatingSegmentDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#SEGMENTREPORTINGScheduleofFinancialInformationaboutCompanysOperatingSegmentDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/SEGMENTREPORTINGScheduleofFinancialInformationaboutCompanysOperatingSegmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_01a020bc-0702-7071-b2f2-bcaf73451552" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a020bc-0702-7222-8ec0-7230e531e6c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a020bc-0702-7071-b2f2-bcaf73451552" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a020bc-0702-7222-8ec0-7230e531e6c4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_01a020bc-0702-733b-9fdf-b15fa73a97b5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_01a020bc-0702-7222-8ec0-7230e531e6c4" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_01a020bc-0702-733b-9fdf-b15fa73a97b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01a020bc-0702-733b-9fdf-b15fa73a97b5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_01a020bc-0702-733b-9fdf-b15fa73a97b5" xlink:to="loc_us-gaap_SegmentDomain_01a020bc-0702-733b-9fdf-b15fa73a97b5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_01a020bc-0702-7ea0-a3dc-7ebafe8b7b93" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_01a020bc-0702-733b-9fdf-b15fa73a97b5" xlink:to="loc_us-gaap_SegmentDomain_01a020bc-0702-7ea0-a3dc-7ebafe8b7b93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_ReportableSegmentMember_01a020bc-0702-7eda-b69a-5d3839505a20" xlink:href="abos-20260930.xsd#abos_ReportableSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_01a020bc-0702-7ea0-a3dc-7ebafe8b7b93" xlink:to="loc_abos_ReportableSegmentMember_01a020bc-0702-7eda-b69a-5d3839505a20" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpensesAbstract_01a020bc-0702-7b1b-9158-e8347f9e4957" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_01a020bc-0702-7071-b2f2-bcaf73451552" xlink:to="loc_us-gaap_OperatingExpensesAbstract_01a020bc-0702-7b1b-9158-e8347f9e4957" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseAbstract_01a020bc-0702-7f57-af02-a565de38349b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResearchAndDevelopmentExpenseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a020bc-0702-7b1b-9158-e8347f9e4957" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseAbstract_01a020bc-0702-7f57-af02-a565de38349b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_ResearchAndDevelopmentExternalCosts_01a020bc-0702-74ab-aa43-d4c8c3e34647" xlink:href="abos-20260930.xsd#abos_ResearchAndDevelopmentExternalCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentExpenseAbstract_01a020bc-0702-7f57-af02-a565de38349b" xlink:to="loc_abos_ResearchAndDevelopmentExternalCosts_01a020bc-0702-74ab-aa43-d4c8c3e34647" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_ResearchAndDevelopmentInternallyManagedCosts_01a020bc-0702-72e2-9f45-50649c246634" xlink:href="abos-20260930.xsd#abos_ResearchAndDevelopmentInternallyManagedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentExpenseAbstract_01a020bc-0702-7f57-af02-a565de38349b" xlink:to="loc_abos_ResearchAndDevelopmentInternallyManagedCosts_01a020bc-0702-72e2-9f45-50649c246634" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_01a020bc-0702-7202-8ba1-d0731c9554cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a020bc-0702-7b1b-9158-e8347f9e4957" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_01a020bc-0702-7202-8ba1-d0731c9554cf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingOtherItemAmount_01a020bc-0702-72f3-b6f4-63b0e53d15dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingOtherItemAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a020bc-0702-7b1b-9158-e8347f9e4957" xlink:to="loc_us-gaap_SegmentReportingOtherItemAmount_01a020bc-0702-72f3-b6f4-63b0e53d15dd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_01a020bc-0702-743c-bc07-7346cfc1a8c1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a020bc-0702-7b1b-9158-e8347f9e4957" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_01a020bc-0702-743c-bc07-7346cfc1a8c1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_01a020bc-0702-7f4e-af54-e95920f44880" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_01a020bc-0702-7b1b-9158-e8347f9e4957" xlink:to="loc_us-gaap_NetIncomeLoss_01a020bc-0702-7f4e-af54-e95920f44880" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.acumenpharm.com/role/COMMITMENTSANDCONTINGENCIESDetails" xlink:type="simple" xlink:href="abos-20260930.xsd#COMMITMENTSANDCONTINGENCIESDetails"/>
  <link:definitionLink xlink:role="http://www.acumenpharm.com/role/COMMITMENTSANDCONTINGENCIESDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsLineItems_01a020bc-0702-78f4-99cc-f1b02116ff1e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCommitmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsTable_01a020bc-0702-73fc-ab73-7129d494e783" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCommitmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_01a020bc-0702-78f4-99cc-f1b02116ff1e" xlink:to="loc_us-gaap_OtherCommitmentsTable_01a020bc-0702-73fc-ab73-7129d494e783" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_01a020bc-0702-7e69-9608-60483f49e084" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_01a020bc-0702-73fc-ab73-7129d494e783" xlink:to="loc_srt_CounterpartyNameAxis_01a020bc-0702-7e69-9608-60483f49e084" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a020bc-0702-7e69-9608-60483f49e084_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_01a020bc-0702-7e69-9608-60483f49e084" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a020bc-0702-7e69-9608-60483f49e084_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a020bc-0702-754e-a5f6-e98bc0c6d234" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_01a020bc-0702-7e69-9608-60483f49e084" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a020bc-0702-754e-a5f6-e98bc0c6d234" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_JCRPharmaceuticalsMember_01a020bc-0702-7f6d-ad16-20bb13d6fffc" xlink:href="abos-20260930.xsd#abos_JCRPharmaceuticalsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_01a020bc-0702-754e-a5f6-e98bc0c6d234" xlink:to="loc_abos_JCRPharmaceuticalsMember_01a020bc-0702-7f6d-ad16-20bb13d6fffc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfArrangementAxis_01a020bc-0702-7b08-bfbf-d9a5368312d8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_01a020bc-0702-73fc-ab73-7129d494e783" xlink:to="loc_us-gaap_TypeOfArrangementAxis_01a020bc-0702-7b08-bfbf-d9a5368312d8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_01a020bc-0702-7b08-bfbf-d9a5368312d8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_01a020bc-0702-7b08-bfbf-d9a5368312d8" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_01a020bc-0702-7b08-bfbf-d9a5368312d8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_01a020bc-0702-7b6f-896e-e281b91cdb5c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_01a020bc-0702-7b08-bfbf-d9a5368312d8" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_01a020bc-0702-7b6f-896e-e281b91cdb5c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember_01a020bc-0702-7df9-a1f8-50363ba96b45" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_01a020bc-0702-7b6f-896e-e281b91cdb5c" xlink:to="loc_us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember_01a020bc-0702-7df9-a1f8-50363ba96b45" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_LonzaLicenseAgreementMember_01a020bc-0702-7b05-ba64-5b020ad2f990" xlink:href="abos-20260930.xsd#abos_LonzaLicenseAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_01a020bc-0702-7b6f-896e-e281b91cdb5c" xlink:to="loc_abos_LonzaLicenseAgreementMember_01a020bc-0702-7b05-ba64-5b020ad2f990" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_01a020bc-0702-7023-83bf-7fc8b77ea845" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_01a020bc-0702-73fc-ab73-7129d494e783" xlink:to="loc_srt_ProductOrServiceAxis_01a020bc-0702-7023-83bf-7fc8b77ea845" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a020bc-0702-7023-83bf-7fc8b77ea845_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_01a020bc-0702-7023-83bf-7fc8b77ea845" xlink:to="loc_srt_ProductsAndServicesDomain_01a020bc-0702-7023-83bf-7fc8b77ea845_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_01a020bc-0702-7837-9327-40ae044f3a3d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_01a020bc-0702-7023-83bf-7fc8b77ea845" xlink:to="loc_srt_ProductsAndServicesDomain_01a020bc-0702-7837-9327-40ae044f3a3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_DevelopmentMember_01a020bc-0702-7745-863c-576f5a1d1f9f" xlink:href="abos-20260930.xsd#abos_DevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a020bc-0702-7837-9327-40ae044f3a3d" xlink:to="loc_abos_DevelopmentMember_01a020bc-0702-7745-863c-576f5a1d1f9f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_SalesAndRoyaltiesMember_01a020bc-0702-7b52-a92b-180df4114105" xlink:href="abos-20260930.xsd#abos_SalesAndRoyaltiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_01a020bc-0702-7837-9327-40ae044f3a3d" xlink:to="loc_abos_SalesAndRoyaltiesMember_01a020bc-0702-7b52-a92b-180df4114105" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_01a020bc-0702-7704-8722-8ea38c2a3985" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_01a020bc-0702-78f4-99cc-f1b02116ff1e" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_01a020bc-0702-7704-8722-8ea38c2a3985" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_CollaborativeArrangementFutureMilestonePayment_01a020bc-0702-744a-a888-04eb62137446" xlink:href="abos-20260930.xsd#abos_CollaborativeArrangementFutureMilestonePayment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_01a020bc-0702-78f4-99cc-f1b02116ff1e" xlink:to="loc_abos_CollaborativeArrangementFutureMilestonePayment_01a020bc-0702-744a-a888-04eb62137446" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_CollaborativeArrangementPaymentOfUpfrontFees_01a020bc-0702-72f0-bcc3-ffc0f0e516c2" xlink:href="abos-20260930.xsd#abos_CollaborativeArrangementPaymentOfUpfrontFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_01a020bc-0702-78f4-99cc-f1b02116ff1e" xlink:to="loc_abos_CollaborativeArrangementPaymentOfUpfrontFees_01a020bc-0702-72f0-bcc3-ffc0f0e516c2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_CollaborativeArrangementRoyaltyPercentage_01a020bc-0702-7535-87d8-585be8e2d711" xlink:href="abos-20260930.xsd#abos_CollaborativeArrangementRoyaltyPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_01a020bc-0702-78f4-99cc-f1b02116ff1e" xlink:to="loc_abos_CollaborativeArrangementRoyaltyPercentage_01a020bc-0702-7535-87d8-585be8e2d711" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_CollaborativeArrangementTerm_01a020bc-0702-7630-a3cc-3b664e008b7d" xlink:href="abos-20260930.xsd#abos_CollaborativeArrangementTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_01a020bc-0702-78f4-99cc-f1b02116ff1e" xlink:to="loc_abos_CollaborativeArrangementTerm_01a020bc-0702-7630-a3cc-3b664e008b7d" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#IndividualsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#ecd_AllIndividualsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_EricSiemersMember_01a110ff-b54e-79e7-82f5-7ee504490ea8" xlink:href="abos-20260930.xsd#abos_EricSiemersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_abos_EricSiemersMember_01a110ff-b54e-79e7-82f5-7ee504490ea8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_RussellBartonMember_01a11106-9940-7230-aa32-c1127303c828" xlink:href="abos-20260930.xsd#abos_RussellBartonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_abos_RussellBartonMember_01a11106-9940-7230-aa32-c1127303c828" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_JamesDohertyMember_01a1115d-8a14-7b1c-a282-f688ac29fdfc" xlink:href="abos-20260930.xsd#abos_JamesDohertyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_abos_JamesDohertyMember_01a1115d-8a14-7b1c-a282-f688ac29fdfc" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/TradingArrangementsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#TradingArrangmentsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/TradingArrangementsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllTradingArrangementsMember" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#ecd_AllTradingArrangementsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_RussellBartonPlan1Member_01a1110c-4ab2-79b7-9c30-ba55dacfbb2d" xlink:href="abos-20260930.xsd#abos_RussellBartonPlan1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllTradingArrangementsMember" xlink:to="loc_abos_RussellBartonPlan1Member_01a1110c-4ab2-79b7-9c30-ba55dacfbb2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_RussellBartonPlan2Member_01a1110c-f592-773d-bb78-36dc9a49aa7b" xlink:href="abos-20260930.xsd#abos_RussellBartonPlan2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllTradingArrangementsMember" xlink:to="loc_abos_RussellBartonPlan2Member_01a1110c-f592-773d-bb78-36dc9a49aa7b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_JamesDohertyPlan1Member_01a1115e-2c82-7df8-8f2a-74b815948e90" xlink:href="abos-20260930.xsd#abos_JamesDohertyPlan1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllTradingArrangementsMember" xlink:to="loc_abos_JamesDohertyPlan1Member_01a1115e-2c82-7df8-8f2a-74b815948e90" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_JamesDohertyPlan2Member_01a1115e-66ba-7546-b0fb-7ec8760f6fb5" xlink:href="abos-20260930.xsd#abos_JamesDohertyPlan2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllTradingArrangementsMember" xlink:to="loc_abos_JamesDohertyPlan2Member_01a1115e-66ba-7546-b0fb-7ec8760f6fb5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_JamesDohertyPlan2SharesOfCommonStockMember_01a1111e-5db8-7064-923e-c7f9ff5b785e" xlink:href="abos-20260930.xsd#abos_JamesDohertyPlan2SharesOfCommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllTradingArrangementsMember" xlink:to="loc_abos_JamesDohertyPlan2SharesOfCommonStockMember_01a1111e-5db8-7064-923e-c7f9ff5b785e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_abos_JamesDohertyPlan2SharesOfCommonStockReceivedUponExerciseOfOptionAwardsMember_01a1111d-af18-716b-ba6c-d71cd2dd6c83" xlink:href="abos-20260930.xsd#abos_JamesDohertyPlan2SharesOfCommonStockReceivedUponExerciseOfOptionAwardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllTradingArrangementsMember" xlink:to="loc_abos_JamesDohertyPlan2SharesOfCommonStockReceivedUponExerciseOfOptionAwardsMember_01a1111d-af18-716b-ba6c-d71cd2dd6c83" xlink:type="arc" order="6"/>
  </link:definitionLink>
</link:linkbase>
