A test's validity refers to how good it is. Validity can be compared with reliability, which refers to how consistent the results would be if the test were given under the same conditions to the same learners.
Face validity refers to how good people think the test is, content validity to how good it actually is in testing what it says it will test.
In the classroom
For a teacher who has to design or choose a formal achievement test for their learners, asking the question ‘Does the test actually test what it aims to test?' offers an important point of reflection. To answer this question positively demands that the test covers the skills, grammar and lexis covered in the course. To offer the learners a test that does not do this gives an inaccurate picture of students' learning, and is unfair.