Chastity, castitas, is set against lust in the scheme of contrary virtues. In the tradition it is not equivalent to celibacy or to abstinence: Aquinas treats chastity as the part of temperance that orders sexual desire according to reason and one's state of life, so that a married person is chaste in the exercise of that state and a celibate in abstaining from it, and the Catechism states this explicitly, describing chastity as the successful integration of sexuality within the person and as required of everyone according to their condition. The distinction is regularly lost in popular usage, where chastity is taken simply to mean not having sex. The related but distinct virtue of continence concerns the restraint of desire against resistance, which Aristotle and Aquinas both place below full virtue, since the fully virtuous person's desires are already ordered.
Augustine's treatise De Bono Coniugali, the good of marriage, supplied much of the later tradition's vocabulary for married chastity, identifying three goods, offspring, fidelity, and sacramental permanence, that make the sexual relationship of spouses good rather than merely tolerated; this framework let medieval theologians hold married intercourse compatible with virtue while still ranking consecrated celibacy, following Paul's preference at 1 Corinthians 7, as the more excellent state, a ranking the Second Vatican Council's later teaching softened without abandoning, by insisting more strongly on the genuine goodness and dignity of married chastity in its own right.