What is Test Case?

Legal Definition
In case law, a test case is a legal action whose purpose is to set a precedent. An example of a test case might be a legal entity who files a lawsuit in order to see if the court considers a certain law or a certain legal precedent applicable in specific circumstances. This is useful, for example, in order to later file similar lawsuits in similar circumstances.

Government agencies sometimes bring test cases in order to confirm or expand their powers.
-- Wikipedia
Legal Definition
Executable test examining inputs, outputs of a system and then provides the details of steps taken, result that should be achieved and elements identified.