The Latin word imperator was originally a title roughly equivalent to commander under the Roman Republic. Later it became a part of the titulature of the Roman Emperors as part of their cognomen. The English word emperor derives from imperator via Old French Empereür. 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 (derived from princeps, from which prince in English is derived) and the dominate.
In Latin, the feminine form of imperator is imperatrix, denoting a ruling female.
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.
In Roman Republican literature and epigraphy, an imperator was a magistrate with imperium (Rivero, 2006). 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 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 45 BC again to Gaius Julius Caesar, in 44 BC to Marcus Iunius 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.