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]