The Collaborative International Dictionary of English v.0.48:

Despot \Des"pot\, n. [F. despote, LL. despotus, fr. Gr.
   despo`ths master, lord, the second part of which is akin to
   po`sis husband, and L. potens. See Potent.]
   1. A master; a lord; especially, an absolute or irresponsible
      ruler or sovereign.
      [1913 Webster]

            Irresponsible power in human hands so naturally
            leads to it, that cruelty has become associated with
            despot and tyrant.                    --C. J. Smith.
      [1913 Webster]

   2. One who rules regardless of a constitution or laws; a
      tyrant.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

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



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

DESPOT. This word, in its most simple and original acceptation, signifies
master and supreme lord; it is synonymous with monarch; but, taken in bad
part, as it is usually employed, it signifies a tyrant. In some states,
despot is the title given to the sovereign, as king is given in others.
Encyc. Lond.