A cloister (from Latin claustrum, "enclosure") is a rectangular open space surrounded by covered walks or open galleries, with open arcades on the inner side, running along the walls of buildings and forming a quadrangle or garth. The attachment of a cloister to a cathedral or church, commonly against a warm southern flank, usually indicates that it is (or once was) part of a monastic foundation, "forming a continuous and solid architectural barrier... that effectively separates the world of the monks from that of the serfs and workmen, whose lives and works went on outside and around the cloister."

Cloistered (or claustral) life is also another name for the life of a monk or nun in the enclosed religious orders; the modern English term enclosure is used in contemporary Catholic church law to mean cloistered, and cloister is sometimes used as a metonymic synonym for monastery.

Historically, the early medieval cloister had several antecedents, the peristyle court of the Greco-Roman domus, the atrium and its expanded version that served as forecourt to early Christian basilicas, and certain semi-galleried courts attached to the flanks of early Syrian churches. Walter Horn suggests that the earliest coenobitic communities, which were established in Egypt by Saint Pachomius, did not result in cloister construction, as there were no lay serfs attached to the community of monks, thus no separation within the walled community was required; Horn finds the earliest prototypical cloisters in some exceptional late fifth-century monastic churches in southern Syria, such as the Convent of Saints Sergios and Bacchos, at Umm-is-Surab (AD 489), and the colonnaded forecourt of the convent of Id-Dêr, but nothing similar appeared in the semieremitic Irish monasteries' clustered roundhouses nor in the earliest Benedictine collective communities of the West.