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Vicarius is a Latin word, meaning substitute or deputy. It is the root and origin of the English word "vicar".
Originally, in ancient Rome, this was an equivalent to the English "vice-" (as in "deputy"), used as part of the title of various officials. Each vicarius was assigned to a specific superior official, after whom his full title was generally completed by a genitive (e.g. vicarius praetoris). At a low level of society, the slave of a slave, possibly hired out to raise money to buy manumission, was a servus vicarius.
Later, in the 290s, the Emperor Diocletian carried out a series of administrative reforms, ushering the period of the Dominate. These reforms also saw the number of Roman provinces increased, and the creation of a new administrative level, the diocese. The dioceses, initially twelve, grouped several provinces, each with its own governor. The dioceses were headed by a vicarius, or, more properly, by a vices agens praefectorum praetorio ("deputy of the praetorian prefect"). An exception was the Diocese of Oriens, which was headed by a comes ("count"). In 370 or 381 Egypt and Cyrenaica were detached from the Diocese of the Orient and made a diocese under an official called the Augustal Prefect.
According to the Notitia dignitatum (an early 5th century imperial chancery document), the vicarius had the rank of vir spectabilis; the staff of a vicarius, his officium, was rather similar to a gubernatorial officium. For example, in the diocese of Hispaniae, the his staff included:
- The princeps (i.e. chief of staff was chosen from among the senior agentes in rebus (couriers or special investigators, 'men of affairs,' from the ministry of the interior headed by the master of the offices), from the salary class of the ducenarii (those earning 200,000 sesterces a year - the highest regular pay grade in the Roman civil service; the highest officials from governor up were not civil service).
- A cornicularius ("chief of staff").
- Two numerarii (chief accountants).
- A commentariensis (keeper of the commentary," the official diary).
- An adiutor (adjutant; literally "helper," an assistant).
- An ab actis ("acts-keeper," archivist).
- A cura epistolarum ("curator of correspondence").
- An unnamed number of subadiuvae ("deputy assistants").
- Various exceptores (lower clerks).
- Singulares et reliquum officium (various menial staff).
Sources and references 
- P. R. C. Weaver, "Vicarius and Vicarianus in the Familia Caesaris" The Journal of Roman Studies 54.1 and 2 (1964:117-128).