A nun is a woman who vows to dedicate her life to religion, typically living under vows of poverty, chastity, and obedience in the enclosure of a monastery or convent. The term is often used interchangeably with religious sisters who do take simple vows but live an active vocation of prayer and charitable work.

In Christianity, nuns are found in the Roman Catholic, Oriental Orthodox, Eastern Orthodox, Lutheran, and Anglican traditions, as well as other Christian denominations. In the Buddhist tradition, female monastics are known as Bhikkhuni, and take several additional vows compared to male monastics (bhikkhus). Nuns are most common in Mahayana Buddhism, but have more recently become more prevalent in other traditions.