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Vicarius is a Latin word, meaning substitute or deputy. It is the root of the English word "vicar."
In ancient Rome, this was the partial title for various officials. Each vicarius was assigned to another superior official, after whom his full title was generally completed. (e.g. Vicarius Praetor) However, there was an exception to this usage in which both the title and position were singular, simply referred to as a vicarius.
During the later period of the Roman Empire known as the "Dominate" or "Tetrarchy", a vicarius was the imperial deputy in charge of a group of Roman provinces called a diocese. This position was introduced under Emperor Diocletian who reformed the Roman Empire, collegially, into the Tetrarchy. Under this system, the empire was ruled by two Augusti (senior emperors) and each was assisted by a Caesar (junior emperor) who acted as a deputy and was groomed to become the next 'Augustus'. Thus a Tetrarchy was a rule by four individuals. (These deputies were soon abolished as a regular institution - leaving only two emperors.) The eastern and western empires were each divided into two large praetorian prefectures.
Each of the four prefectures 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 province with its own governor.
For example, the western empire was divided into Italia (Italy & Africa) and Galliae (the Gauls). Galliae contained the four dioceses of Britain, Hispania, Viennensis and Gaul. The diocese of Britain was divided into four (and later five) provinces and the vicarius of Britain, based in London, oversaw the governors of each of the British provinces.
The various prefectures, dioceses and provinces are listed systematically in their hierarchical groupings in the article Roman province.
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