in some Christian churches, a territorial area administered by a bishop. The word originally referred to a governmental area in the Roman Empire, governed by an imperial vicar. The secular diocese was subdivided into provinces, each with its own governor; but, in the ecclesiastical adaptation of the system, the province became the larger territorial unit, administered by a metropolitan bishop and subdivided into dioceses.

The original unit of ecclesiastical administration was the parish, which in the Eastern Orthodox church still remains the designation of the area administered by the bishop, whereas the diocese is the larger area administered by the patriarch. The use of these terms was still fluid in the West in the 9th century; but, by the 13th century, diocese meant the territory administered by a bishop.

In the Roman Catholic church (Roman Catholicism) only the pope can divide or merge dioceses or create new ones. All dioceses are divided into parishes, each with its own church; dioceses are also sometimes divided into rural deaneries, which contain several parishes.

In the Church of England (England, Church of), during the 16th, 19th, and 20th centuries, new dioceses were created by statute by dividing existing ones. Each diocese is subdivided into parishes, which are grouped under rural deaneries and archdeaconries.

The other Protestant churches have abandoned the term in favour of such terms as district, conference, or even synod.