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From 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]