1 The function, office, position, or role in which one acts. 2A legal qualification, such as age, that determines one's ability to do something that has legal consequences (such as making a contract or getting married). Also called legal capacity. 3 The mental ability to perceive, understand, and appreciate all relevant facts, to make a rational decision based thereon, and to understand the nature and effect of one's actions. See also sane. criminal capacity. The mental ability required to su...