Tsar
From Memory Alpha, the free Star Trek reference
Tsar (or Czar) is the Russian word designating the formal title of Emperor. Peter the Great was the tsar of Russia during the late 17th and early 18th century. (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?")
