Apostasy (; 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. 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.