Heresy (/) is any belief or theory that is strongly at variance with established beliefs or customs, in particular the accepted beliefs of a church or religious organization. A heretic is a proponent of such claims or beliefs. Heresy is distinct from both apostasy, which is the explicit renunciation of one's religion, principles or cause, and blasphemy, which is an impious utterance or action concerning God or sacred things.
The term is usually used to refer to violations of important religious teachings, but is used also of views strongly opposed to any generally accepted ideas. It is used in particular in reference to Christianity, Judaism, and Islam.
In certain historical Christian, Islamic and Jewish cultures, among others, espousing ideas deemed heretical has been and in some cases still is subjected not merely to punishments such as excommunication, but even the death penalty.