Post-Roman Use
After the Roman empire collapsed in the West in the 5th century, Latin continued to be used as the language of learning and diplomacy for some centuries. The Roman emperors of this period (referred to by modern historians as the Byzantine emperors) were referred to as imperatores in Latin texts, while the word basileus (king) was used in Greek.
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 the monarchy's claim that it was the successor to the Byzantine emperors, Peter the Great imported the Latin word directly into Russian and styled himself imperator (Императоръ). 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"), which they had begun to use c. 1480 to likewise assert their contention to be the heirs to the Byzantine state (see: Third Rome.) Reigning female Russian rulers were styled imperatritsa.
Napoleon famously adopted the title for himself and 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 in the West ceased. It was thereafter used only historically or as a Latin translation for certain continuing titles of non-European cultures, such as Japan.
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