Source : Webster's Revised Unabridged Dictionary (1913)

Usurpation \U`sur*pa"tion\, n. [L. usurpatio ? making use,
   usurpation: cf. F. usurpation.]
   1. The act of usurping, or of seizing and enjoying; an
      authorized, arbitrary assumption and exercise of power,
      especially an infringing on the rights of others;
      specifically, the illegal seizure of sovereign power; --
      commonly used with of, also used with on or upon; as, the
      usurpation of a throne; the usurpation of the supreme
      power.

Source : WordNet®

usurpation
     n 1: entry to another's property without right or permission
          [syn: {trespass}, {encroachment}, {violation}, {intrusion}]
     2: wrongfully seizing and holding (an office or powers) by
        force (especially the seizure of a throne or supreme
        authority); "a succession of generals who ruled by
        usurpation"