Monastery (plural: monasteries) denotes the building, or complex of buildings, that houses a room reserved for prayer (e.g. an oratory) as well as the domestic quarters and workplace(s) of monastics, whether monks or nuns, and whether living in community or alone (hermits).

The earliest extant use of the term monastērion is by the first century AD Jewish philosopher Philo (On The Contemplative Life, ch. III).

Monasteries may vary greatly in size – a small dwelling accommodating only a hermit, or in the case of communities anything from a single building housing only a one senior and two or three junior monks or nuns, to vast complexes and estates housing tens or hundreds.

In English usage, the term "monastery" is generally used to denote the buildings of a community of monks. The name convent tends to be used (inaccurately) for the buildings accommodating female monastics (nuns). (The term "nunnery" for the latter is outmoded and considered offensive). It may also be used to reflect the Latin usage for houses of friars, more commonly called a friary, or for communities of teaching or nursing Religious Sisters. Various religions may use these terms in more specific ways.

In most religions the life inside monasteries is governed by community rules that stipulates the gender of the inhabitants and requires them to remain celibate and own little or no personal property. The degree to which life inside a particular monastery is socially separate from the surrounding populace can also vary widely; some religious traditions mandate isolation for purposes of contemplation removed from the everyday world, in which case members of the monastic community may spend most of their time isolated even from each other. Others are focused on interacting with the local communities in order to provide some service, such as teaching, medical care, or evangelism. Some monastic communities are only occupied seasonally, depending both on the traditions involved and the local weather, and people may be part of a monastic community for periods ranging from a few days at a time to almost an entire lifetime.

The life within the walls of a monastery may be supported in several ways: by manufacturing and selling goods, often agricultural products such as cheese, wine, beer, liquor, and jellies; by donations or alms; by rental or investment incomes; and by funds from other organizations within the religion which in the past has formed the traditional support of Monasteries. However, today Christian monastics have updated and adapted themselves to modern society by offering computer services, accounting services, management as well as modern hospital administration in addition to running schools, colleges and universities.

For a discussion of the history and development of the life inside hermit cottages see monasticism and abbey.