A heresy (Greek, "choice") is a belief or opinion that is contrary to orthodox (accepted and defined) religious doctrine. The term is mainly used in Christianity, Islam, and Judaism.
A person who believes or teaches a heresy is called a heretic; a leader of a heretical teaching or group is known as a heresiarch.
Heterodoxy is a somewhat different term; it usually refers to a teaching that departs from the orthodox but falls short of heresy.
- “heresy.” Cross, F. L. and E. A. Livingstone, The Oxford Dictionary of the Christian Church (Oxford University Press, USA). "...the Greek word αἵρεσις, denoting 'choice' or 'thing chosen', from which the term is derived..."