The cloister has not always signified a structure that encloses monastery or convent like it is thought of today. The English word "cloister" comes from the Latin word "clausura", and clausura comes from the Latin verb "claudere" or "to shut up". Rather the cloister refers to the actual law or rule of a convent or monastery that shuts off a certain group of religious by lifestyle. It most simply refers to two or three (or a small group) of religious who live together in a permanent residence under a certain rule. Depending on the specific rule the laws can be more or less strict. For example in Western Europe women were not allowed to enter a men's monastery at all or even in some cases the inner part of a nun's convent. While in Eastern Europe an Africa there existed "double monasteries" where nuns and monks practiced the same rule under the same roof. Also in some cases pious women, virigins and widows, were allowed to live with clerics and monks. It was only later that the actual enclosing structure developed with the architecture in the Middle Ages to create what today is thought of as a cloister.
"Cloister." The Catholic Encyclopedia. Vol. 4. New York: Robert Appleton Company, 1908.
25 Apr. 2012 <http://www.newadvent.org/cathen/04060a.htm>.