What is Test Validity?

Legal Definition
Test validity is the extent to which a test (such as a chemical, physical, or scholastic test) accurately measures what it is supposed to measure. In the fields of psychological testing and educational testing, “validity refers to the degree to which evidence and theory support the interpretations of test scores entailed by proposed uses of tests.” Although classical models divided the concept into various "validities" (such as content validity, criterion validity, and construct validity), the currently dominant view is that validity is a single unitary construct.

Validity is generally considered the most important issue in psychological and educational testing because it concerns the meaning placed on test results. Though many textbooks present validity as a static construct, various models of validity have evolved since the first published recommendations for constructing psychological and education tests. These models can be categorized into two primary groups: classical models, which include several types of validity, and modern models, which present validity as a single construct. The modern models reorganize classical "validities" into either "aspects" of validity or "types" of validity-supporting evidence
-- Wikipedia
Legal Definition
Measure of the degree of accuracy of a tat procedure.