Content validity is the degree to which the content of the test matches the content domain of the target construct.
For example, a test of ability to add two-digit numbers should cover the full range of combinations of digits (10 to 99). A test with only one-digit numbers, or only even numbers, would not have good coverage of the content domain.
Content validity evidence typically involves subject matter experts evaluating test items against the test specifications. Such expert-based testing of content validity is distinguished from the simpler notion of face validity.