According to the Standards1, "Validity refers to the degree to which evidence and theory support the interpretations of test scores entailed by the proposed uses of tests. Validity is, therefore, the most fundamental consideration in developing and evaluating tests." (p.9).
In other words, when we administer a test and make use of the resulting scores, we must ask ourselves whether or not it is appropriate to make such use of the scores. We answer that question by asking ourselves if we have sufficient evidence to support the inferences we make and the actions we take based upon a test score. Professional standards coupled with testing expertise guide us in how we answer that question.
1American Educational Research Association, American Psychological Association, National Council on Measurement in Education (1999). Standards for Educational and Psychological Testing. Washington, DC: AERA