A test case in software engineering is a set of conditions or variables under which a tester will determine whether an application or software system meets specifications. The mechanism for determining whether a software program or system has passed or failed such a test is known as a test oracle. In some settings an oracle could be a requirement or use case. It may take many test cases to determine that a software program or system is functioning correctly. Test cases are often referred to as test scripts, particularly when written. Written test cases are usually collected into test suites.
Formal, requirement-based test cases
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.