thesaurus.maths.org

Validity is a term to describe a measure instrument (like questionary), or test that accurately measures way it is supposed to measure.
It is easy to make a valid measure instrument for to measure an individuals height, but it is more complicated to make an instrument to measure his intelligence.
It is also quite difficult to measure what is difficult.

A formula often only works or gives sensible values under certain conditions or for certain values of its arguments. Inside this range it is said to be valid.
More generally, a piece of mathematics may be said to be valid, to work, only in certain circumstances.