Tsar is the title for the supreme ruler of Bulgaria in 913–1422, later usage in 1908–1946, the supreme ruler of Serbia in 1346—1371 and the emperor of Russia from 1546 until 1721. Since 1721 the title of Russian emperors was "imperator", but the word "tsar" remained in common use until the Russian Revolution 1917. It is also spelled Tzar, Czar, and Csar, and is the Eastern European word for Caesar, that can be translated to Emperor. The system of government used during the Tsardom of Russia and the Russian Empire was called Tsarist autocracy, also known as Tsarism.