Acceptance testing is a final stage of testing that is performed on a system prior to the system being delivered to a live environment. Systems subjected to acceptance testing might include such deliverables as a software system or a mechanical hardware system. Acceptance tests are generally performed as "black box" tests. Black box testing means that the tester uses specified inputs into the system and verifies that the resulting outputs are correct, without knowledge of the system's internal workings.
User acceptance testing (UAT), is the term used when the acceptance tests are performed by the person or persons who will be using the live system once it is delivered. If the system is being built or developed by an external supplier, this is sometimes called customer acceptance testing (CAT). The UAT or CAT acts as a final confirmation that the system is ready for go-live. A successful acceptance test at this stage may be a contractual requirement prior to the system being signed off by the client.