Since the time of Diocletian (end of the 3rd century), it was a large administrative unit constituted by up to 16 provinces. The Empire was separated into 12 dioceses (later 15). The diocese was governed by praetor vicarius who was subjected to the praefectus.
Between the 4th and 6th centuries, Rome became more and more Christian. At the same time, the older administrative structure began to crumble. 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 Eastern Orthodox churches began to define their administrative structure, they relied on the older Roman terminology to describe administrative units and hierarchy. Thus today in the Roman Catholic and some Protestant churches, a diocese is an administrative territorial unit governed by a bishop. Once the diocese became more associated with the office of bishop, the alternative bishopric also came into use. It is also called a see. The Eastern Orthodox church uses the term eparchy for their administrative units.
There are currently about 569 Catholic archdioceses and 2014 dioceses in the world.