I am struggling to find this on the web.
Here is an extract form the Prince2 Glossary, which defines Acceptance Criteria. A prioritised list of criteria that the final product(s) must meet before the customer will accept them; a measurable definition of what must be done for the final product to be acceptable to the customer. They should be defined as part of the Project Brief and agreed between customer and supplier no later than the project initiation stage. They should be documented in the Project Initiation Document.