What is Validity?

Legal Definition
In logic, an argument is valid if and only if it takes a form that makes it impossible for the premises to be true and the conclusion nevertheless to be false. It is not required that a valid argument have premises that are actually true, but to have premises that, if they were true, would guarantee the truth of the argument's conclusion. A formula is valid if and only if it is true under every interpretation, and an argument form (or schema) is valid if and only if every argument of that logical form is valid.
-- Wikipedia
Legal Definition
This term is used to signify legal sufficiency, in contradistinction to mere regularity. "An official sale, an order, judgment or decree may be regular,—the whole practice in reference to its entry may be correct,—but still invalid, for reasons going behind the regularity of its forme," Sharpleigh v. Surdam, 1 Flip. 487, Fed. Cas. No, l2,7lL
-- Black's Law Dictionary