In some Christian churches, the diocese is an administrative territorial unit governed by a bishop, sometimes also referred to as a bishopric or episcopal see, though more often the term episcopal see means the office held by the bishop. In the Roman Catholic Church, an important diocese, governed by an Archbishop is called an archdiocese (usually due to size, historical significance, or both). As of 2003, there are about 569 Roman Catholic archdioceses and 2014 dioceses in the world.
Some Protestant churches such as the Church of England have inherited this diocesan structure directly, during the Protestant Reformation.
In the later organization of the Roman Empire, the increasingly subdivided provinces were administratively associated in a larger unit, the diocese. (Latin dioecesis, from a Greek term meaning "administration").
The Catholic Church directly inherited this Roman structure of authority during the 5th and 6th centuries, as each bishop fully assumed the role of the former Roman praefectus . The transfer was facilitated by the Christian practice of setting the areas of ecclesiastical administration very exactly coinciding with those of the civil administration: in modern times, many an ancient diocese, though later divided among several dioceses, has preserved the boundaries of a long-vanished Roman administrative division. See further information concerning bishops in civil government at the entry Bishop.
In the Roman Empire
The earliest use of "diocese" as an administrative unit was in the Greek-speaking East, applied for instance to three districts— Cibyra, Apamea and Synnada— that were added to the province of Cilicia in the time of Cicero, who mentions the fact in his familiar letters (EB 1911). The word, an equivalent to a tax-collecting district, came to be applied to the territory itself.
In the reorganization of the empire that was begun by Diocletian and carried through by Constantine, the empire was divided into twelve dioceses, of which the largest, Oriens , included sixteen provinces, and the smallest, of Britain, included four. A list of Roman dioceses as the finally were in 395 CE can be found at the entry Roman province.
Each diocese was governed by a praetor vicarius who was subjected to the praefectus . Between the 4th and 6th centuries, as the older administrative structure began to crumble, the position of the bishops in the Christianized Empire of Late Antiquity expanded to fill the vacuum. The senatorial aristocracy, especially in the provinces, remained a source of local authority. By this time, however, that authority was often vested in the spiritual office of bishop. It is therefore of little surprise that, as the Catholic and later the Eastern Orthodox churches began to define their administrative structure, they relied on the older Roman terminology to describe administrative units and hierarchy, and ecclesiastical and secular authorities blurred together. In the Eastern Empire, this became fundamental doctrine: see Caesaropapism.
Christian usage in the modern sense of the sphere of a bishop's jurisdiction became commonplace only within the consciously "classicizing" structure of the Carolingian empire in the 9th century, but the usage had been taking over from the much earlier parochia ("parish") from the surfacing of the Christian authority structure in the 4th century (see EB 1911).