Russian Emperor

Tsar /zɑːr/ or /tsɑːr/ (Old Church Slavonic: ц︢рь [usually written thus with a title] or цар, цaрь), also spelled tzar, csar, or czar, is a title used to designate certain Slavic monarchs or supreme rulers. As a system of government in the Tsardom of Russia and the Russian Empire, it is known as , or Tsarism. The term is derived from the Latin word Caesar, which was intended to mean "Emperor" in the European medieval sense of the term—a ruler with the same rank as a Roman emperor, holding it by the approval of another emperor or a supreme ecclesiastical official (the Pope or the Ecumenical Patriarch)—but was usually considered by western Europeans to be equivalent to king, or to be somewhat in between a royal and imperial rank.

Occasionally, the word could be used to designate other, secular, supreme rulers. In Asia and Russia 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.

"Tsar" was the official title in the following states:

The first ruler to adopt the title tsar was Simeon I of Bulgaria.Simeon II, the last Tsar of Bulgaria, is the last person to have borne the title Tsar as well as being the last surviving person to do so.

...
Wikipedia