Acceptance Criteria
In plain terms
Conditions that define when a story is complete and correct.
Going deeper
Acceptance criteria specify the testable requirements a feature must satisfy to be accepted, clarifying scope and expectations. They define 'done' for a story.
Under the hood
Often written as scenarios (given-when-then); drive testing and shared understanding.
Examples
User stories, given-when-then, testable conditions, sign-off, BDD.
Think of it like...
Like a checklist the work must satisfy before the customer signs off.