Not valid; not true, correct, acceptable or appropriate.
Suffering from disability or illness.
Intended for use by an invalid.
To make invalid or affect with disease.
To exempt from (often military) duty because of injury or ill health.
Based on reasoning, evidence and good judgement.
Properly substantiated.
Of a binary relation: such that every non-empty subset of the relation's domain has a minimal element with respect to the relation.