Test validity is one of the most important ideas in educational assessments. Anyone preparing a test must be certain that the test is valid. This is particularly true given the extreme importance of standardized tests in our educational system today.
Test validity is the degree to which a test measures what it is supposed to measure. For example, the SAT is supposed to measure how ready a student is for college and how well they are likely to do once in college. It is a valid test if higher scores actually do correlate with better results in college. At the level of tests that we make as individual teachers, we must be sure that our tests measure the skills and knowledge that we have actually taught our students in the time leading up to the test.