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):

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

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "czar":
   Dalai Lama, Holy Roman Emperor, Inca, Kaiser, Simon Legree,
   absolute monarch, absolute ruler, all-powerful ruler, ardri,
   arrogator, autarch, autocrat, bey, cacique, caesar, cham,
   commissar, despot, dictator, disciplinarian, driver, duce,
   hard master, kaid, khan, martinet, mikado, negus, oligarch,
   oppressor, padishah, pendragon, pharaoh, rig, sachem, sagamore,
   shah, sheikh, shogun, slave driver, stickler, tenno, tycoon,
   tyrant, usurper, warlord



Bouvier's Law Dictionary, Revised 6th Ed (1856):

CZAR. A title of honor which is assumed by the emperor of all the Russias.
See Autocracy.