A test case is a set of conditions or variables used to determine if a software application or system behaves as expected. It typically includes inputs, execution steps, and the expected outcome. Test cases are essential in software development to ensure that features work correctly and to identify any defects. In software testing, each test case is designed to validate a specific function or requirement of the application. They help testers systematically check the functionality of the software, ensuring that it meets the specifications outlined in the requirements document. Properly written test cases can improve the efficiency of the testing process and enhance the overall quality of the software product.