Tsar (Tzar, Czar, or Csar Bulgarian: цар, Serbian: цар, Russian:  царь (help·info), Ukrainian: цар) is a title used to designate certain European Slavic monarchs or supreme rulers. As a system of government in the Tsardom of Russia and Russian Empire, it is known as Tsarist autocracy, or Tsarism. The term is derived from the Latin word Caesar, which meant "Emperor" in the European medieval sense of the term - a ruler who claims the same rank as a Roman emperor, with the approval of another emperor or a supreme ecclesiastical official (the Pope or the Ecumenical Patriarch).

Occasionally, the word could be used to designate other, secular, supreme rulers. In Russia and Bulgaria the imperial connotations of the term were blurred with time, due to the medieval translations of the Bible, and, by the 19th century, it had come to be viewed as an equivalent of King.