Officium (ancient Rome)

Officium is a Latin word with various meanings in ancient Rome, including "service", " duty", "courtesy", "ceremony" and the like. It commonly also referred to the office of a magistrate and his sometimes numerous staff, each of whom was called an officialis.
The Notitia Dignitatum gives us uniquely detailed information, stemming from the very imperial chanceries, on the composition of the officia of many of the leading court, provincial, military and certain other officials of the two Roman empires AD. While the details vary somewhat according to rank, from West to East and/or in particular cases, in general the leading staff would be about as follows :

Princeps officii was the chief of staff, permanent secretary or chef de cabinet
Cornicularius was a military title, for an administrative deputy of various generals etc.
Adiutor seems to have been the chief assistant, or adjutant
Commentariensis was the keeper of "commentaries", an official diary
Ab actis was the keeper of records, the archivist
Numerarius seems to have been the receiver of taxes
Subadiuva seems to have been a general assistant
Cura epistolarum was the curator of correspondence
Regerendarius may have been a registrar
Exceptor seem to have been a secretary
Singularius has been called a notary, but the word can also refer to a bodyguard

Below those "dignities", there were often several hundred subordinate staff, sometimes slaves or freedmen, performing day-to-day administrative duties, not deemed worthy of any more detailed mention. They are only referred to collectively, by various terms in the plural, such as cohortalini.