A test case is a set of conditions or variables under which a tester will determine whether an application, software system, or one of its features is working as it was originally established for. It is a specific test that is designed to verify if a feature or functionality of the system is working correctly. A test case typically includes test inputs, the expected results, and any other information needed to run the test. It is a part of the testing process and helps to ensure that the system meets its requirements and works as intended.