The Latin word imperator ( ) derives from the stem of the verb , meaning 'to order, to command'. It was originally employed as a title roughly equivalent to commander under the Roman Republic. Later it became a part of the titulature of the Roman Emperors as their praenomen. The English word emperor derives from imperator via . The Roman emperors themselves generally based their authority on multiple titles and positions, rather than preferring any single title. Nevertheless, imperator was used relatively consistently as an element of a Roman ruler's title throughout the Principate and the Dominate.
Imperatores in the ancient Roman Kingdom
When Rome was ruled by kings, to be able to rule, the king had to be invested with the full regal authority and power. So, after the comitia curiata, held to elect the king, the king also had to be conferred the imperium.
Imperatores in the Roman Republic
In Roman Republican literature and epigraphy, an imperator was a magistrate with imperium. But also, mainly in the later Roman Republic and during the late Republican civil wars, imperator was the honorific title assumed by certain military commanders. After an especially great victory, an army's troops in the field would proclaim their commander imperator, an acclamation necessary for a general to apply to the Senate for a triumph. After being acclaimed imperator, the victorious general had a right to use the title after his name until the time of his triumph, where he would relinquish the title as well as his imperium.
Since a triumph was the goal of many politically ambitious Roman commanders, Roman Republican history is full of cases where legions were bribed to call their commander imperator. The title of imperator was given firstly to Aemilius Paulus in 189 BC for his campaigns against the Lusitanians from 191-189 BC then to Lucius Cornelius Sulla in 95 BC as governor or pro consul of the Cilician province for his returning of Ariobarzanes I to the Cappadocia throne. Then in 90 BC to Lucius Julius Caesar, in 84 BC to Gnaeus Pompeius Magnus, in 60 BC to Gaius Julius Caesar, relative of the previously mentioned Lucius Julius Caesar, in the 50s to Gaius Julius Caesar (in Gaul), in 45 BC again to Gaius Julius Caesar, in 43 BC to Decimus Junius Brutus, and in 41 BC to Lucius Antonius (younger brother and ally of the more famous Marcus Antonius). In 15 AD Germanicus was also imperator during the empire (see below) of his adoptive father Tiberius.
Imperator as an imperial title
After Augustus established the Roman Empire, the title imperator was generally restricted to the emperor, though in the early years of the empire it would occasionally be granted to a member of his family. As a permanent title, imperator was used as a praenomen by the Roman emperors and was taken on accession. After the reign of Tiberius, the act of being proclaimed imperator was transformed into the act of imperial accession. In fact, if a general was acclaimed by his troops as imperator, it would be tantamount to a declaration of rebellion against the ruling emperor. At first the term continued to be used in the Republican sense as a victory title but attached to the de facto monarch and head of state, rather than the actual military commander. The title followed the emperor's name along with the number of times he was acclaimed as such, for example IMP V ("imperator five times"). In time it became the title of the de facto monarch, pronounced upon (and synonymous with) their assumption.
As a title imperator was generally translated into Greek as autokrator ("one who rules himself," also sometimes used as a translation for Roman dictators). This was necessarily imprecise as it lost the nuances of Latin political thought contrasting imperium with other forms of public authority. Nevertheless, this title (along with sebastos for augustus) was used in Greek-language texts for Roman emperors from the establishment of the empire.
In the east, the title continued to be used into the Byzantine period, though to a lesser, and much more ceremonial, extent. In most Byzantine writings, the Greek translation "Autokrator" is preferred, but "Imperator" makes an appearance in Constantine IV's mid 7th century mosaic in the Basilica of Sant'Apollinare in Classe, and on various 9th century lead seals.