Tsar (or Czar) is a Slavic title derived from Caesar and analogous to emperor, used historically in Bulgaria, Serbia and Russia.
Peter the Great was the tsar of Russia during the late 17th and early 18th century, later also adopting the title of emperor. (TOS: "The Trouble with Tribbles")
After meeting Apollo, Pavel Chekov referred to himself as "the tsar of all the Russias". (TOS: "Who Mourns for Adonais?")