A diocese is the district or see under the supervision of a bishop. It is divided into parishes.

An archdiocese is more significant than a diocese. An archdiocese is presided over by an archbishop whose see may have or had importance due to size or historical significance. The archbishop may have metropolitan authority over any other (then 'suffragan') bishops and their dioceses within his ecclesiastical province.

This structure of church governance is known as episcopal polity.

A diocese also may be referred to as a bishopric or episcopal see, though strictly the term episcopal see refers to the domain of ecclesiastical authority officially held by the bishop, and the term bishopric to the post of being bishop.

As of January 2009^[update] there are 630 Catholic archdioceses (including 13 patriarchates, two catholicates, 536 metropolitan archdioceses, 79 single archdioceses) and 2,167 dioceses in the world.

In the Eastern Catholic Churches (which recognize papal authority and are a part of the larger Catholic Church), the equivalent unit is called an eparchy.