Apostasy (/əˈpɒstəsi/; Greek: ἀποστασία (apostasia), "a defection or revolt") is the formal disaffiliation from, or abandonment or renunciation of a religion by a person. It can also be defined within the broader context of embracing an opinion contrary to one's previous beliefs. One who commits apostasy (or who apostatizes) is known as an apostate. The term apostasy is used by sociologists to mean renunciation and criticism of, or opposition to, a person's former religion, in a technical sense and without pejorative connotation.
The term is occasionally also used metaphorically to refer to renunciation of a non-religious belief or cause, such as a political party, brain trust, or a sports team.
Apostasy is generally not a self-definition: few former believers call themselves apostates because of the negative connotation of the term.
Many religious groups and some states punish apostates; this may be the official policy of the religious group or may simply be the voluntary action of its members. Such punishment may include shunning, excommunication, verbal abuse, physical violence, or even execution. Examples of punishment by death for apostates can be seen under the Sharia Law found in certain Islamic countries.
The American sociologist Lewis A. Coser (following the German philosopher and sociologist Max Scheler) defines an apostate as not just a person who experienced a dramatic change in conviction but "a man who, even in his new state of belief, is spiritually living not primarily in the content of that faith, in the pursuit of goals appropriate to it, but only in the struggle against the old faith and for the sake of its negation."