The Collaborative International Dictionary of English v.0.48:

Tsar \Tsar\, n.
   The title of the emperor of Russia. See Czar.
   [1913 Webster] Tsarina

The Collaborative International Dictionary of English v.0.48:

czar \czar\ (z[aum]r), n. [Russ. tsare, fr. L. Caesar C[ae]sar;
   cf. OPol. czar, Pol. car. ]
   A king; a chief; the title of the emperor of Russia. [Written
   also tsar and tzar.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

tsar
    n 1: a male monarch or emperor (especially of Russia prior to
         1917) [syn: czar, tsar, tzar]