formal disaffiliation from or abandonment or renunciation of a religion

Apostasy is a term used in a religious context, for leaving one's religion. Usually it means moving to another religion. Someone who does this is called an apostate. Apostasy is different from heresy: A heretic only questions one (or more) of the traditions and beliefs of their religion. An apostate questions all of them.

Logo of The Campaign for Collective Apostasy in Spain, calling for defection from the Catholic Church
States in the world, where there is the death penalty for apostasy. In many of these states, Islam is one of the main religions.

The word was first used in Christianity: In the Roman Catholic Church the punishment for apostasy is excommunication. Today, the word is used mostly in the context of apostasy in Islam.

Sometimes the term is used as a metaphor to refer to changing a non-religious belief or cause, such as a political party or a sports team.

Apostasy in Islam;

A person who rejects, underestimates, mocks or disrespects religious orders defined as fard or sunnah in fiqh or speaks words called "alfaz-i kufr" is called apostate. According to the Sharia law, the penalty is death.