In church governance, a diocese or bishopric is the ecclesiastical district under the jurisdiction of a bishop.

In the later organization of the Roman Empire, the increasingly subdivided provinces were administratively associated in a larger unit, the diocese (Latin dioecesis, from the Greek term διοίκησις, meaning "administration").

Christianity was given legal status in 313 with the Edict of Milan. Churches began to organize themselves into dioceses based on the civil dioceses, not on the larger regional imperial districts. These dioceses were often smaller than the provinces. Christianity was declared the Empire's official religion by Theodosius I in 380. Constantine I in 318 gave litigants the right to have court cases transferred from the civil courts to the bishops. This situation must have hardly survived Julian, 361–363. Episcopal courts are not heard of again in the East until 398 and in the West in 408. The quality of these courts was low, and not above suspicion as the Bishop of Alexandria Troas found that clergy were making a corrupt profit. Nonetheless, these courts were popular as people could get quick justice without being charged fees. Bishops had no part in the civil administration until the town councils, in decline, lost much authority to a group of 'notables' made up of the richest councilors, powerful and rich persons legally exempted from serving on the councils, retired military, and bishops post-AD 450. As the Western Empire collapsed in the 5th century, bishops in Western Europe assumed a larger part of the role of the former Roman governors. A similar, though less pronounced, development occurred in the East, where the Roman administrative apparatus was largely retained by the Byzantine Empire. In modern times, many dioceses, though later subdivided, have preserved the boundaries of a long-vanished Roman administrative division. For Gaul, Bruce Eagles has observed that "it has long been an academic commonplace in France that the medieval dioceses, and their constituent pagi, were the direct territorial successors of the Roman civitates."

Modern usage of 'diocese' tends to refer to the sphere of a bishop's jurisdiction. This became commonplace during the self-conscious "classicizing" structural evolution of the Carolingian Empire in the 9th century, but this usage had itself been evolving from the much earlier parochia ("parish"; Late Latin derived from the Greek παροικία paroikia), dating from the increasingly formalized Christian authority structure in the 4th century.

Dioceses ruled by an archbishop are commonly referred to as archdioceses; most are metropolitan sees, being placed at the head of an ecclesiastical province. In the Catholic Church, some are suffragans of a metropolitan see or are directly subject to the Holy See.

The term "archdiocese" is not found in Catholic canon law, with the terms "diocese" and "episcopal see" being applicable to the area under the ecclesiastical jurisdiction of any bishop. If the title of archbishop is granted on personal grounds to a diocesan bishop, his diocese does not thereby become an archdiocese.^{[better source needed]}

The Canon Law of the Catholic Church defines a diocese as "a portion of the people of God which is entrusted to a bishop for him to shepherd with the cooperation of the presbyterium, so that, adhering to its pastor and gathered by him in the Holy Spirit through the gospel and the Eucharist, it constitutes a particular church in which the one, holy, catholic, and apostolic Church of Christ is truly present and operative."

Also known as particular churches or local churches, dioceses are under the authority of a bishop. They are described as ecclesiastical districts defined by geographical territory. Dioceses are often grouped by the Holy See into ecclesiastical provinces for greater cooperation and common action among regional dioceses. Within an ecclesiastical province, one diocese can be designated an "archdiocese" or "metropolitan archdiocese", establishing centrality within an ecclesiastical province and denoting a higher rank. Archdioceses are often chosen based on their population and historical significance. All dioceses and archdioceses, and their respective bishops or archbishops, are distinct and autonomous. An archdiocese has limited responsibilities within the same ecclesiastical province assigned to it by the Holy See.