A nun is a woman who vows to dedicate her life to religious service and contemplation, 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 the wider society.

In Christianity, nuns are found in the Catholic, Oriental Orthodox, Eastern Orthodox, Lutheran, and Anglican and some Presbyterian 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.

In the Catholic tradition, there are many religious institutes of nuns and sisters (the female equivalent of male monks or friars), each with its own charism or special character. Traditionally, nuns are members of enclosed religious orders and take solemn religious vows, while sisters do not live in the papal enclosure and formerly took vows called "simple vows".

As monastics, nuns living within an enclosure historically commit to recitation of the full Divine Office throughout the day in church, usually in a solemn manner. They were formerly distinguished within the monastic community as "choir nuns", as opposed to lay sisters who performed upkeep of the monastery or errands outside the cloister. This last task is still often entrusted to women, called "externs", who live in the monastery, but outside the enclosure. They were usually either oblates or members of the associated Third Order, often wearing a different habit or the standard woman's attire of the period.

In general, when a woman enters a religious order or monastery she first undergoes a period of testing life for six months to two years called a postulancy. If she, and the order, determine that she may have a vocation to the life, she receives the habit of the order (usually with some modification, normally a white veil instead of black, to distinguish her from professed members) and undertakes the novitiate, a period (that lasts one to two years) of living the life of the religious institute without yet taking vows. Upon completion of this period she may take her initial, temporary vows. Temporary vows last one to three years, typically, and will be professed for not less than three years and not more than six. Finally, she will petition to make her "perpetual profession", taking permanent, solemn vows.

In the branches of the Benedictine tradition, (Benedictines, Cistercians, Camaldolese, and Trappists, among others) nuns take vows of stability (that is, to remain a member of a single monastic community), obedience (to an abbess or prioress), and conversion of life (which includes poverty and celibacy). In other traditions, such as the Poor Clares (the Franciscan Order) and the Dominican nuns, they take the threefold vows of poverty, chastity and obedience. These are known as the 'evangelical counsels' as opposed to 'monastic vows' proper. Most orders of nuns not listed here follow one of these two patterns, with some Orders taking an additional vow related to the specific work or character of their Order (for example, to undertake a certain style of devotion, praying for a specific intention or purpose).

Cloistered nuns (Carmelites, for example) observe "papal enclosure" rules, and their nunneries typically have walls separating the nuns from the outside world. The nuns rarely leave (except for medical necessity or occasionally for purposes related to their contemplative life) though they may receive visitors in specially built parlors, often with either a grille or half-wall separating the nuns from visitors. They are usually self-sufficient, earning money by selling jams, candies or baked goods by mail order, or by making liturgical items (such as vestments, candles, or hosts to be consecrated at Mass for Holy Communion).

They often undertake contemplative ministries – that is, a community of nuns is often associated with prayer for some particular good or supporting the missions of another order by prayer (for instance, the Dominican nuns of Corpus Christi Monastery in the Bronx, New York, pray in support of the priests of the Archdiocese of New York). Yet religious sisters can also perform this form of ministry, e.g., the Maryknoll Missionary Sisters have small houses of contemplative sisters, some in mission locations, who pray for the work of the priests, brothers, and other sisters of their congregation, and since Vatican II have added retreat work and spiritual guidance to their apostolate; the Sister Disciples of the Divine Master are also cloistered sisters who receive visitors and pray in support of their sister congregation, the Daughters of St. Paul in their media ministry.