A test case in software engineering is a set of conditions or variables under which a tester will determine if a requirement or use case upon an application is partially or fully satisfied. It may take many test cases to determine that a requirement is fully satisfied.
Test cases are often incorrectly referred to as test scripts. Test scripts are lines of code used mainly in automation tools.
Written test cases are usually collected into test suites.
In order to fully test that all the requirements of an application are met, there must be at least one test case for each requirement unless a requirement has sub-requirements. In that situation, each sub-requirement must have at least one test case. This is frequently done using a traceability matrix. Some methodologies, like RUP, recommend creating at least two test cases for each requirement. One of them should perform positive testing of requirement and other should perform negative testing. Written test cases should include a description of the functionality to be tested, and the preparation required to ensure that the test can be conducted.
What characterizes a formal, written test case is that there is a known input and an expected output, which is worked out before the test is executed. The known input should test a precondition and the expected output should test a postcondition.
A variation of test cases are most commonly used in acceptance testing. Acceptance testing is done by a group of end-users or clients of the system to ensure the developed system meets the requirements specified. User acceptance testing is usually differentiated by the inclusion of happy path or positive test case.