What is Invalid?

Legal Definition
In a physical sense, it is that which is wanting force; in a figurative sense, it signifies that which has no effect.
-- Bouviers Law Dictionary
Legal Definition
Vain; inadequate to its purpose; not of binding force or legal efficacy; lacking in authority or obligation. Hood v. Perry, 75 Ga. 312; State v. Casteel, 110 Ind. 174, 11 N. E. 219; Mutual Ben. L. Ins. Co. v. Winne, 20 Mont. 20, 49 Pac. 446.
-- Black's Law Dictionary
Legal Definition
Void; not valid.
-- Ballentine's Law Dictionary