The term czar (also spelled tsar) is the Russian version of Caesar, the family name of Julius Caesar and the first emperors of Rome. Russians first used the word to refer to the emperors of the Byzantine Empire, the eastern part of the Roman Empire. Both the Byzantines and the Russians belonged to the Eastern Orthodox branch of Christianity. The Byzantine emperor was considered the head of the Orthodox Christian world.

After the fall of the Byzantine Empire in 1453, the princes of Moscow (the capital of Russia) became leaders of the Orthodox world. In 1472 Ivan III of Moscow married the niece of the last Byzantine emperor. Because of this marriage, Ivan III and the rulers who followed him claimed to be successors of the Byzantine emperors. Ivan III's grandson, Ivan IV, was the first Russian ruler to use the title of czar. Known as Ivan the Terrible, he had vast power and ruled harshly. Later czars had similar qualities.

In 1721 Peter I, also called Peter the Great, stopped using the title of czar as part of his efforts to make Russia more modern. Even so, Russia's emperors continued to be called czars until the last of them, Nicholas II, was removed from the throne. Nicholas and his entire family were killed by revolutionaries in 1918 so that no descendants could claim the throne in the future. (See also Revolution, Russian.)