Apostasy (from Greek αποστασία, meaning a defection or revolt, from απο, apo, "away, apart", στασις, stasis, "standing") is a term generally employed to describe the formal renunciation of one's religion, especially if the motive is deemed unworthy. In a technical sense, as used sometimes by sociologists without the pejorative connotations of the word, the term refers to renunciation and criticism of, or opposition to one's former religion. One who commits apostasy is an apostate, or one who apostatises. Apostasy is generally not a self-definition: very few former believers call themselves apostates and they generally consider this term to be a pejorative. One of the possible reasons for this renunciation is loss of faith, another is the failure of alleged religious indoctrination or brainwashing.
Many religious movements consider it a vice (sin), a corruption of the virtue of piety in the sense that when piety fails, apostasy is the result.
Several religious groups and even some states punish apostates. Apostates may be shunned by the members of their former religious group or worse. This may be the official policy of the religious group or may happen spontaneously, due in some sense to psycho-social factors as well. The Catholic Church may in certain very limited circumstances respond to apostasy by excommunicating the apostate, while the writings of both ancient Judaism (Deuteronomy 13:6-10) and Islam (al-Bukhari, Diyat, bab 6) demand the death penalty for apostates.
The reliability of the testimonies of apostates is an important and controversial issue in the study of apostasy in cults and new religious movements.
The difference between apostasy and heresy is that the latter refers to rejection or corruption of certain doctrines, not to the complete abandonment of one's religion. Heretics claim to still be following a religion (or to be the "true followers"), whereas apostates reject it.
The term is also used to refer to renunciation of belief in a cause other than religion, particularly in politics. Conversely, some atheists and agnostics use the term "deconversion" to describe loss of faith in a religion. Self-described "Freethinkers" and those who may view traditional religion negatively may see it as gaining rationality and respect for the scientific method.
Other terms to describe leaving a faith and the associated process are treated in religious disaffiliation.