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]

The Devil's Dictionary (1881-1906):

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