The word "tsar" (or czar) meant the supreme title for a ruler of Russia and some other Slav countries. The etymology of this word goes back to the time of Julius Caesar. Caesar was an essential title of Roman and Byzantine Emperors as successors of Julius Caesar. This word came into the Slavonic language from the Gothic. Ivan the Terrible, known as the Grand Prince of all Russia was the first one who was formally proclaimed the tsar of all Russia. Using this title he wanted to emphasize the succession of Byzantine traditions. Peter the Great was the last one named by this title "tsar" since it was changed to Emperor.
Learn more about etymology of Russian words from your Moscow tour guide.