Apostasy

Apostasy is the act of leaving a religion, either to convert to another religion, or to atheism, deism, or agnosticism.

In some theocratic countries apostasy is punishable by death. In most religions, apostasy is punishable by eternal damnation, a fate worse than death.

Sources defining and discussing the standard meaning of apostasy:
Merriam-Webster - apostasy (merriam-webster.com)
Dictionary.com apostasy (dictionary.com)
TheFreeDictionary.com apostasy (thefreedictionary.com)
Merriam-Webster - apostate (merriam-webster.com)
Dictionary.com apostate (dictionary.com)
Your Dictionary: Apostasy (yourdictionary.com)
Wikipedia - apostasy (wikipedia.org)
Wikipedia - Apostasy in Judaism (Wikipedia.org)
Wikipedia - Apostasy in Islam (Wikipedia.org)
PewResearchCenter Which countries still outlaw apostasy and blasphemy? by Angelina E. Theodorou (pewreasearch.org)
Apologetics Index: Apostasy (apologeticsindex.org) includes apostasy from cults and cultic responses to those who have apostatized from their ranks
Mormon Think: apostasy (mormonthink.com) Article discussing phenomenon of apostasy from the ranks of the Latter-Day Saints, and how the LDS Church defines "apostasy".
Baker's Evangelical Dictionary of Biblical Theology: Apostasy. Mark W. Karlberg (biblestudytools.com)
International Standard Bible Encyclopedia: Apostasy; Apostate. Dwight M. Pratt (biblestudytools.com)
Catholic Encyclopedia: Apostasy (newadvent.org) This article defines multiple meanings of apostasy.

See also Great Apostasy.