A test case is like a specification for a test. When we point to the "test case", we try to define the situation surrounding the test and its execution. A set of instructions and/or data for testing some part of a product in some way.

ISTQB: A set of preconditions, inputs, actions (where applicable), expected results and postconditions, developed based on test conditions.

A test case by itself has no value, it is the content that makes it possibly interesting or valuable. This is related to counting test cases.