A test case is a set of conditions or steps designed to exercise a particular aspect of an application, system or component. It is used in software testing to verify that the code under test meets specific requirements and behaves as expected. A test case typically includes input data, expected output, and a predefined result or error message based on whether the actual output matches the expected output or not. Test cases are written with the goal of identifying defects or errors in the software being tested so that they can be fixed before the application is released to users.