Apostasy (Greek απο, apo, "away, apart", στασις, stasis, "standing") is the renunciation of one's religion. It is customarily considered a vice, a corruption of the virtue of piety in the sense that when piety fails, apostasy is the result. However, most converts to a new religion can also be considered apostates from a previous belief. The word is also used to refer to renunciation of belief in a cause other than religion.
One who is guilty of apostasy is an apostate, or one who apostatises. In Islam, along with certain other faiths, the punishment for apostasy is death.
See Virtue and Great Apostasy.