Religious fanaticism, or religious extremism, is a pejorative designation used to indicate uncritical zeal or obsessive enthusiasm that is related to one's own, or one's group's, devotion to a religion – a form of human fanaticism that could otherwise be expressed in one's other involvements and participation, including employment, role, and partisan affinities. Historically, the term was applied in Christian antiquity to denigrate non-Christian religions, and subsequently acquired its current usage with the Age of Enlightenment.

Fanaticism is a belief or behavior involving uncritical zeal or an obsessive enthusiasm.