Meaning
The explicit set of predefined requirements and conditions that a feature must satisfy to be accepted by the product owner.
Three examples from work
Requirement check
DevAre the acceptance criteria finalized for the password reset flow?
POYes, the link must expire after fifteen minutes and require multi-factor verification.
QA validation
QAThe search filter fails the third acceptance criterion on Safari.
DevThanks for catching that; I'll fix the CSS flex layout bug.
Client sign-off
ClientWe tested the export module and all acceptance criteria are met.
PMWonderful, we will proceed with production deployment tonight.
Often abbreviated as 'AC'. Clear AC prevents misunderstandings between developers, testers, and product managers.