The Collaborative International Dictionary of English v.0.48:

Dictator \Dic*ta"tor\, n. [L.]
   1. One who dictates; one who prescribes rules and maxims
      authoritatively for the direction of others. --Locke.
      [1913 Webster]

   2. One invested with absolute authority; especially, a
      magistrate created in times of exigence and distress, and
      invested with unlimited power.
      [1913 Webster]

            Invested with the authority of a dictator, nay, of a
            pope, over our language.              --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dictator
    n 1: a speaker who dictates to a secretary or a recording
         machine
    2: a ruler who is unconstrained by law [syn: dictator,
       potentate]
    3: a person who behaves in a tyrannical manner; "my boss is a
       dictator who makes everyone work overtime" [syn:
       authoritarian, dictator]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "dictator":
   Simon Legree, absolute monarch, absolute ruler, all-powerful ruler,
   arrogator, autarch, authoritarian, authoritative, autocrat, caesar,
   commissar, czar, despot, disciplinarian, doctrinaire, driver, duce,
   hard master, martinet, oligarch, oppressor, overlord, pharaoh,
   slave driver, stern, stickler, strong man, tyrant, usurper,
   warlord



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

DICTATOR, civil law. A Magistrate at Rome invested with absolute power. His
authority over the lives and fortunes of the citizens was without bounds.
His office continued but for six months. Hist. de la Jur. h.t.; Dig. l, 2,
18; Id. 1, 1, 1.



The Devil's Dictionary (1881-1906):

DICTATOR, n.  The chief of a nation that prefers the pestilence of
despotism to the plague of anarchy.