The Collaborative International Dictionary of English v.0.48:

Autocrat \Au"to*crat\, n. [Gr. ?; ? self + ? strength, ? strong:
   cf. F. autocrate. See Hard, a.]
   1. An absolute sovereign; a monarch who holds and exercises
      the powers of government by claim of absolute right, not
      subject to restriction; as, Autocrat of all the Russias (a
      title of the Czar).
      [1913 Webster]

   2. One who rules with undisputed sway in any company or
      relation; a despot.
      [1913 Webster]

            The autocrat of the breakfast table.  --Holmes.
      [1913 Webster] Autocratic

WordNet (r) 3.0 (2006):

autocrat
    n 1: a cruel and oppressive dictator [syn: tyrant, autocrat,
         despot]

Moby Thesaurus II by Grady Ward, 1.0:

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