Princeps (plural: principes) is a Latin word meaning "first in time or order; the first, foremost, chief, the most eminent, distinguished, or noble; the first man, first person". As a title, "princeps" originated in the Roman Republic wherein the leading member of the Senate was designated princeps senatus. It is primarily associated with the Roman emperors as an unofficial title first adopted by Augustus (reigned 27 BCE - 14 CE) in 23 BCE. Its use in this context continued until the reign of Diocletian (r. 284 - 305 CE) at the end of the third century. He preferred the title of dominus, meaning "lord" or "master". As a result, the Roman Empire from Augustus to Diocletian is termed the "principate" (principatus) and from Diocletian onwards as the "dominate" (dominatus). Other historians define the reign of Augustus to Severus Alexander (r. 222 - 235) as the Principate, and the period afterwards as the "Autocracy".
Princeps was also used as the second part of various other military titles, such as Decurio princeps, Signifer princeps (among the standard-bearers). See also Principalis (as in Optio principalis): NCO.
Princeps is also the (official) short version of Princeps officii, the chief of an officium (the office staff of a Roman dignitary).
Princeps civitatis ("First Citizen") was an official title of a Roman Emperor as the title determining the leader in Ancient Rome at the beginning of the Roman Empire. It created the principate Roman imperial system.
This usage of "princeps" derived from the position of Princeps senatus, the "first among equals" of the Senate. The princeps senatus (plural principes senatus) was the first member by precedence of the Roman Senate, and his opinion would usually be asked first in senatorial debates.
It was first given as a special title to Caesar Augustus in 27 BC, who saw that use of the titles rex (king) or dictator would create resentment amongst senators and other influential men, who had earlier demonstrated their disapproval by supporting the assassination of Julius Caesar. While Augustus had political and military supremacy, he needed the assistance of his fellow Romans to manage the Empire. In his Res Gestae, Augustus claims auctoritas for the princeps (himself).
Various official titles were associated with the Roman Emperor. These titles included imperator, Augustus, Caesar, and later dominus (lord) and basileus (the Greek word for "sovereign"). The word Emperor is derived from the Roman title "imperator", which was a very high, but not exclusive, military title until Augustus began to use it as his praenomen.
The Emperor Diocletian (284-305), the father of the Tetrarchy, was the first to stop referring to himself as "princeps" altogether, calling himself "dominus" (lord, master), thus dropping the pretense that emperor was not truly a monarchical office. The period when the emperors that called themselves princeps ruled--from Augustus to Diocletian--is called "the Principate", Diocletian's rule began "the Dominate" period.
Ancient Rome knew another kind of "princely" principes too, like "princeps iuventutis" ("the first amongst the young"), which in the early empire was frequently bestowed on eligible successors to the emperor, especially from his family. It was first given to Augustus' maternal grandsons Gaius and Lucius.
"Princeps" is the root and Latin rendering of modern words as the English title and generic term prince (see that article, also for various equivalents in other languages), as the Byzantine version of Roman law was the basis for the legal terminology developed in feudal (and later absolutist) Europe.