Imperator (Latin ) designated in the Roman Republic, originally the support of a military force ( imperium ). From the late 3rd century BC ( Scipio Africanus ) but the term was increasingly used specifically for a military commander, whom his soldiers had proclaimed after a victory to the Emperor ( acclamation ). This honorary title was extinguished when the emperor returned to Rome and the pomerium exceeded. Usually, this was done in a solemn triumph, to which legitimate the Imperatore title. Many Roman generals described themselves in their provinces but as emperors, without having been proclaimed so.

Octavian, the future Augustus, took about 39/38 BC Emperor instead of his original name Gaius as first names ( praenomen imperatoris ). The subsequent emperors renounced beginning of it, but from Nero Emperor was again fixed date of imperial titulary, usually abbreviated to Imp and directly followed by the title of Caesar, then the individual name components and the title Augustus (example: Imp Caesar M. Aurelius Commodus Antoninus Augustus ). In a military success of their legions, the Emperor received, even if they were not personally involved to continue the imperious acclamation, which also appeared with their count in the titulary ( imp. III, so for the third time ). Until the end of late antiquity remained Imperator part of the imperial titulary; so was still called Imperator Caesar Flavius ​​Justinian I. Justinian Augustus. Only under Heraclius was allowed to 625 the title Imperator ( Emperor gr ) fall.

From the title of Imperator was in many languages ​​, eg in English ( emperor ) and French ( Empereur ), the term for an emperor.