n. An abandonment of what one has professed; a total desertion of, or departure from, one's faith, principles, or party.n. In theology, a total abandonment of the Christian faith.n. In Rom. Cath. eccles. law: A persistent rejection of ecclesiastical authority by a member of the church. An abandonment without permission of the religious order of which one is a member. A renunciation of the clerical profession by one who has received major orders.n. In medicine, same as apostasis.n. Also spelled apostacy.