Definition of invalid

• a. - Of no force, weight, or cogency; not valid; weak.
• a. - Having no force, effect, or efficacy; void; null; as, an invalid contract or agreement.
• a. - A person who is weak and infirm; one who is disabled for active service; especially, one in chronic ill health.
• n. - Not well; feeble; infirm; sickly; as, he had an invalid daughter.
• v. t. - To make or render invalid or infirm.
• v. t. - To classify or enroll as an invalid.