A Tester works with the Customer and an Analyst to define AcceptanceCriteria.
AcceptanceCriteria specify, as a collection of Scenario, what needs to be achieved if the Behaviour of the Feature is to be considered finished.
AcceptanceCriteria are sometimes, incorrectly, referred to as SuccessCriteria. They differ in that in some cases AcceptanceCriteria can describe how the system fails as well as how it succeeds.