A Roman or civil diocese (Latin: dioecēsis, from the Greek: διοίκησις, "administration") was one of the administrative divisions of the later Roman Empire, starting with the Tetrarchy. It formed the intermediate level of government, grouping several provinces and being in turn subordinated to a praetorian prefecture.
The earliest use of "diocese" as an administrative unit was in the Greek-speaking East. Three districts, Cibyra, Apamea, and Synnada, were added to the Province of Cilicia in the time of Cicero, who mentioned the fact in his epistles. The word "diocese", which at that time denoted a district pertinent for the collection of taxes, was applied to the territory per se.
The reorganization of the Roman Empire known as the Tetrarchy was initiated by Emperor Diocletian in the AD 290s. He divided the Provinces into smaller, more compact, and easily governed units, with a greatly increased bureaucracy. The Provinces were in turn grouped into 12 dioceses, each headed by a vicarius ("dioceseos"), i. e. a deputy to the Praefectus praetorio ("Praetorian Prefect"). The Dioceses themselves were grouped into 4 huge prefectures, each one headed by a powerful Praefectus praetorio. Under the Tetrarchy, each of the 2 senior Emperors, the Augusti, governed a praetorian prefecture. The largest diocese by number of provinces, not area, was the Diocese of the East, which included 16 provinces, while the smallest, the Diocese of Britain, comprised only 4 provinces.
After Emperor Constantine I instituted definite territorial praetorian prefectures circa AD 326-8, the Dioceses functioned as the intermediate echelon between the Provinces and the Prefectures, although the hierarchy did not function rigidly: provincial governors could appeal directly to the Praefectus praetorio or Emperor and vice versa. The objective was to govern, coordinate provincial administration, regulate courts, secure revenue, and incorporate the fiscal offices the Sacrae Largitiones, i. e., the Imperial treasury which collected monetary taxes and res privata, i. e., the private property of the Emperor, because the Prefects determined the universal budget for all departments. The Vicarii were the principal judicial and financial officials of the regions with general supervision, but restricted power. In the West, the dioceses were disbanded as Roman power and jurisdiction receded, but in the East, they survived. Seeing their role as somewhat ineffectual, Emperor Justinian I abolished most of the dioceses in his great reform of the AD 530s, preferring to strengthen the authority of provincial governors. This practice was extended to the recovered territories of Italy and Africa, where Justinian preferred to install Praetorian Prefects to directly govern the respective provinces.
Between the 4th and 6th centuries, as the older administrative structure began to crumble, the role of the bishops in the western lands of the Empire enabled those lands and their peoples to maintain a semblance of civilization as the authority of Rome vanished. The senatorial aristocracy, especially in the provinces, continued in many places to serve as sources of local authority to complement the authority assumed by the Church. In Late Antiquity, political power often came to be vested in the spiritual offices of the bishops in each region. This transfer of authority from secular officials to ecclesiastical leaders was natural in that, because of the close integration of the secular and ecclesiastical leadership in the Empire, the areas of ecclesiastical administration always coincided with those of the Roman civil administration.
Therefore, as the Catholic and Eastern Orthodox churches began to define their administrative structures, they relied on the older Roman terminology and methods to describe administrative units and hierarchy, which often caused the division between ecclesiastical and secular authority to disappear. In the Eastern Empire, this became fundamental doctrine: see Caesaropapism and State church of the Roman Empire.
A millennium later this process would be somewhat repeated when the Ottoman Empire conquered the Eastern Roman Empire (see Christianity and Judaism in the Ottoman Empire) and the eastern bishops assumed political roles as the Roman civil structure was stripped away. In modern times, many an ancient diocese, though later divided among several dioceses, has preserved the boundaries of a long-vanished Roman administrative division.