Tsar or czar (Bulgarian цар, Russian: царь (help·info), Ukrainian: цар, in Serbian: цар / car, in scientific transliteration respectively car' and car), occasionally spelled csar or tzar in English, is a Slavic term with Bulgarian origins used to designate certain monarchs. The first ruler to adopt the title tsar was Simeon I of Bulgaria 
Originally, the title Czar (derived from Caesar) meant Emperor in the European medieval sense of the term, that is, 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, non-Christian, supreme rulers. In Russia and Bulgaria the imperial connotations of the term were blurred with time and, by the 19th century, it had come to be viewed as an equivalent of King.
"Tsar" was the official title of the supreme ruler in the following states:
Bulgaria in 913–1018, in 1185–1422 and in 1908–1946
Serbia in 1346–1371
Russia from about 1547 until 1721 (replaced in 1721 by imperator, but remained in common usage until 1917).
Simeon Saxe-Coburg-Gotha, the last Tsar of Bulgaria, is the only living person who bore the Slavonic title Tsar.
So it's interchangable?
Maybe we should Americanize it even more... Xar!
I was going to do a Xena Warrior Princess thing here. You think it would be easy to find a pic of her, and you would be wrong. Apparently every half decent looking chick (and a few that were not) from the 1990's dressed up like Xena and it's on the internet now.
Like, I can't believe we had the same idea for a costume here in 1995.
OK, what was I talking about again?