sound; just; well-founded

a valid reason.

producing the desired result; effective

a valid antidote for gloom.

having force, weight, or cogency; authoritative.

legally sound, effective, or binding; having legal force

a valid contract.

Logic . (of an argument) so constructed that if the premises are jointly asserted, the conclusion cannot be denied without contradiction.

Archaic . robust; well; healthy.

adjective

invalid , unacceptable , unreal , unsound , wrong

adjective

accurate , attested , authentic , authoritative , binding , bona fide , cogent , compelling , conclusive , confirmed , convincing , credible , determinative , efficacious , efficient , good , in force , irrefutable , just , kosher * , lawful , legal , legit , legitimate , logical , official , original , persuasive , potent , powerful , proven , pure , solid , sound , stringent , strong , substantial , telling , tested , true , trustworthy , ultimate , unadulterated , unanswerable , uncorrupted , weighty , well-founded , well-grounded , tight , faithful , certified , effective , genuine , healthy , justifiable , realistic , reasonable , right , satisfactory , satisfying , sufficient , verified