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The Latin word Princeps (plural: principes) means exactly 'a prime'.

This article is devoted to a number of specific historical meanings the word took, by far the most important of which follows first.

Roman Emperor

Princeps (in this sense usually translated as "First Citizen") was an official title of a Roman Emperor, by some historians seen as the title determining the Emperor in Ancient Rome.

The word "Princeps" derived from Princeps Senatus ("Primus inter pares" of the Senate). It was first given to the Emperor Augustus in 23 BC, who circumspectly 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).

For a comprehensive list of other official Roman titles used for the office of emperor see Roman Emperor. These titles included imperator, Augustus, Caesar, and later dominus ("lord") and basileus (the Greek word for "sovereign"). The word Emperor itself 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 (285-305), 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", while no later than under Diocletian 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.

Roman administration

Main article: officium

Princeps is also the (official) short version of Princeps officii, the chief of an officium (the office staff of a Roman dignitary) -

Military

• See Principes (legionary heavy infantry soldier)
• centurio(n) in command of a unit or administrative office.
• Princeps ordinarius vexillationis: centurion in command of a vexillatio (detachment).
• Princeps peregrinorum ("commander of the foreigners"): centurion in charge of troops in the castra peregrina (military base at Rome for personnel seconded from the provincial armies)
• Princeps prior: Centurion commanding a manipulus (unit of two centuries) of principes (legionary heavy infantry).
• Princeps posterior: deputy to the Princeps prior
• Princeps praetorii : centurion attached to headquarters.

Princeps was also used as defining 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.

Nobiliary legacy

"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.

Non-Roman meaning

"Princeps" is also the name of an obsolete genus of Swallowtail butterflies (now merged with the genus Papilio).

Fiction

• "Princeps" is the title for the 'captain' of a Titan, a massive humanoid war machine in the tabletop wargame Warhammer 40,000.
• In the book series Codex Alera by Jim Butcher, "Princeps" is the title given to the crown prince of the empire of Alera.