In the Roman Catholic, Eastern Orthodox, Anglican, and some other Christian churches, Diocese is the term used for a cluster of parishes—usually organized geographically—that is overseen by a bishop. The number of parishes in a diocese can range from several to fifty or more, depending upon denomination. This form of organization dates from the Early Church era. Both the word itself and the concept of a regional administrative unit were derived from the Roman Empire's hierarchy of political subdivisions.
It can also be referred to as a "bishopric" or a "see" (as in the "Holy See" i.e. the Vatican)