What is an Acceptance Criteria?
Acceptance criteria are the specific, testable conditions a deliverable must meet before the customer accepts it. They convert "done" from an opinion into a checklist — and they're agreed before the work, because criteria negotiated after delivery are just arguments with formatting.
In agile they live on every user story; in contracts they're the line between payment and dispute. Vague criteria ("user-friendly," "robust") are future conflicts wearing professional clothing.
Worked example
A story says "customer can reset password." Its acceptance criteria: reset email arrives within 60 seconds, link expires after 30 minutes, old password stops working immediately, three failed attempts lock the account for 15 minutes. Five testable sentences — and the demo either passes them or it doesn't, with nothing left to debate.