Tsar is a title for the supreme ruler in several countries.
In the Russian Empire, the title was used from 1546 until 1721. Starting in 1721, the title of Russian emperors was imperator, but the word tsar remained in common use until the Russian Revolution in 1917. The last tsar to rule Russia was Nicholas II, who replaced Alexander III in 1894. The system of government used during the Tsardom of Russia and the Russian Empire was called Tsarist autocracy, also known as Tsarism.