Tsar (or Czar) was an Earth title derived from Caesar and analogous to emperor.
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")
The 19th century song "Abdul Abulbul Amir" mentioned many brave men serving the tsar as soldiers. (TNG: "Brothers")
After meeting Apollo in 2267, Pavel Chekov sarcastically referred to himself as "the tsar of all the Russias". (TOS: "Who Mourns for Adonais?")