Vicarius

Vicarius is a Latin word, meaning substitute or deputy.In ancient Rome, this was the title of varies officials, each specifically assigned to second an individual superior, after whom his full title was generally completed.

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• Hence Vicarius is the etymological root of the modern word "vicar."
• However, one specific -ironically atypical- application was by far the most important one, in absolute way (i.e. without specification) referred to as Vicarius.

Under the Dominate, a vicarius was the imperial 'vicar' (deputy) in charge of a group of Roman provinces called diocese. The post was introduced under Diocletian who reformed the Roman Empire, collegially, as Tetrarchy. Under this system, the empire was ruled by two Augusti, each assisted by a deputy and successor styled Caesar (these deputies were soon abolished as a regular feature - only two emperors were left). The eastern and western empires remained each divided in two large praetorian prefectures.

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Each of the four prefectures, which were not abolished, was run by a Praetorian Prefect (originally the 'chief of staff' of each of the four imperial tetrarchs) and contained several dioceses and the vicarius of each diocese was responsible for a number of provinces, each with its own governor.

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• For example, Galliae ('the Gauls') was a praetorian prefecture i the western empire (the other being called Italy & Africa) containing the dioceses of Britain, of the Iberian pensinula and of two halves of Gaul. Britain was divided into four (later five) provinces and the Vicar of Britain, based in London, oversaw the governors of the British provinces.
  

The various prefectures, dioceses and provinces are listed systematically in hierarchical groupings in the article Roman province

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Dominate: The Dominate was the 'despotic' last of the two phases of government in the ancient Roman Empire between its establishment in 27 BC and the formal date of the collapse of the Western Empire in AD 476....

Roman province: In Ancient Rome, a province (Latin, provincia, pl. provinciae) was the largest territorial and administrative unit of the empire's foreign possessions (those beyond the Italian peninsula). The word province in modern English has its origins in the term used by the Romans....

Diocese: In some Christian churches, the diocese is an administrative territorial unit governed by a bishop, sometimes also referred to as a bishopric or episcopal see, though more often the term episcopal see means the office held by the bishop. In the Roman Catholic Church, an important diocese, governed ...