The Collaborative International Dictionary of English v.0.48:

Usurper \U*surp"er\, n.
   One who usurps; especially, one who seizes illegally on
   sovereign power; as, the usurper of a throne, of power, or of
   the rights of a patron.
   [1913 Webster]

         A crown will not want pretenders to claim it, not
         usurpers, if their power serves them, to possess it.
                                                  --South.
   [1913 Webster]

WordNet (r) 3.0 (2006):

usurper
    n 1: one who wrongfully or illegally seizes and holds the place
         of another [syn: usurper, supplanter]

Moby Thesaurus II by Grady Ward, 1.0:

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



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

USURPER, government. One who assumes the right of government by force,
contrary to and in violation of the constitution of the country. Toull. Dr.
Civ. n. 32. Vide Tyranny,