Strictly speaking, a cloister is a square or quadrilateral enclosure, surrounded by covered walkways. Cloisters are sometimes attached to a college and more often to a cathedral, but are usually associated with monasteries and convents. Because cloisters were frequently found in monastic complexes and religious retreats, the term cloistered came to be associated with being sheltered or kept away from the outside world.

In a monastery, the cloister is usually the area in the middle of the principle buildings, affording easy communication between them, and serving as the center of activity. Educational instruction often took place in medieval cloisters; the areas also served as places of recreation, and they were even customary places for burial. Larger monasteries had more than one cloister.

The earliest cloisters were basically open arcades, often with sloping wooden roofs. In southern climates, this style remained common as the Middle Ages progressed, but in England the style altered to a range of windows, sometimes provided with glass, allowing light into a vaulted ambulatory.

Today, the branch of the Metropolitan Museum of Art devoted to medieval art and architecture is called The Cloisters.