Apostasy (/əˈpɒstəsi/; Greek: ἀποστασία apostasía, "a defection or revolt") is the formal disaffiliation from, abandonment of, or renunciation of a religion by a person. It can also be defined within the broader context of embracing an opinion that is contrary to one's previous religious beliefs.[1] One who undertakes apostasy is known as an apostate. Undertaking apostasy is called apostatizing (or apostasizing – also spelled apostacizing). The term apostasy is used by sociologists to mean the renunciation and criticism of, or opposition to, a person's former religion, in a technical sense, with no pejorative connotation.

Occasionally, the term is also used metaphorically to refer to the renunciation of a non-religious belief or cause, such as a political party, social movement, or sports team.

Apostasy is generally not a self-definition: few former believers call themselves apostates due to the term's negative connotation.

Many religious groups and some states punish apostates; this may be the official policy of a particular religious group or it may simply be the voluntary action of its members. Such punishments may include shunning, excommunication, verbal abuse, physical violence, or even execution.[2]