In Latin, the feminine form of Imperator is Imperatrix, denoting a ruling female.
In the Roman Republic, imperator was the 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 50 BC to Marcus Tullius Cicero, 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 Tiberius Augustus Germanicus was also imperator during the empire (see below) of his most famous relative Tiberius Augustus.
After Augustus established the hereditary, one-man rule in Rome that we refer to as the Roman Empire, the title imperator was generally restricted to the emperor, though 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.
In the imperial period, the term did continue to be used in the Republican sense as a victory title; however, it could only be granted to the emperor, even if he had not commanded the victorious army in person. 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").
The title imperator was generally translated into Greek as autokrator. This title (along with sebastos for augustus) was used in Greek-language texts by eastern Roman emperors until the seventh century, when basileus began to supplant it.
After the Roman empire collapsed in the West in the fifth century, Latin continued to be used as the language of learning and diplomacy for some centuries. The Eastern Roman, or Byzantine emperors, were referred to as imperatores in Latin texts.
After 800, the imperator was used (in conjunction with augustus) as a formal Latin title in succession by the Carolingian and German Holy Roman Emperors until 1806 and by the Austrian Emperors until 1918.
In 1721, as part of his drive to both westernize the Russian Empire and assert his imperial status as a successor to the Byzantine emperors, Peter the Great imported the Latin word directly into Russian and styled himself imperator (IМПЕРАТОРЪ). The style remained the official one for all his successors down to the end of the Russian Empire in 1917, though the Russian rulers continued to be colloquially known as tsar (a word derived from "Caesar"). Reigning female Russian rulers were styled imperatritsa.
After the Napoleonic wars, the number of emperors in Europe proliferated, but Latin began to fall out of use for all but the most ceremonial situations. Still, in those rare cases in which a European monarch's Latin titles were used, imperator was used as a translation for emperor. Famously, after assuming the title Emperor of India, British monarchs would follow their signatures with the initials RI, standing for rex imperator ("king-emperor"). George VI of the United Kingdom was the last European ruler to claim an imperial title; when he abdicated as Emperor of India in 1948, the last active use of the title imperator ceased.
It is not clear when the feminine form of the Latin term imperator originated or was used for the first time. It usually indicates a reigning monarch, and is thus used in the Latin version of titles of modern reigning Empresses.
Likewise, when Fortuna is qualified "imperatrix mundi" in the Carmina Burana there's no implication of any type of consort - the term describes (the Goddess or personified) Fortune "ruling the world".
In Christian context, Imperatrix became a laudatory address to the Virgin Mary, in diverse forms at least since the Middle Ages — for example, she is sometimes called "Imperatrix angelorum" ("regnant of the angels").
Imperator Online is also an Alternate Earth MMORPG by Mythic Entertainment, the makers of DAOC, one set in a future world where Ancient Rome never fell. Minor changes at important moments in Roman history create an extremely different timeline for Earth, leading to an interstellar Roman Respublica and thousands of years of galactic Pax Romana. However, the game's production was cancelled in July 2005.
Imperator is also the title taken by that member of the Rhon taking on the position and responsibilities of the Military Key, the Commander-in-Chief of the Skolian Imperialate, from the Saga of the Skolian Empire by science fiction author Catherine Asaro. The title is the same regardless of whether taken by a male or female, and is semi-hereditary.
Pavo muticus imperator ("Imperator" or Indo-Chinese Green Peafowl or Dragonbird) is a subspecies of the endangered Green Peafowl that has been suggested to be a distinct species.