Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

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