The Collaborative International Dictionary of English v.0.48:

Usurpation \U`sur*pa"tion\, n. [L. usurpatio ? making use,
   usurpation: cf. F. usurpation.]
   [1913 Webster]
   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.
      [1913 Webster]
      [1913 Webster]

            He contrived their destruction, with the usurpation
            of the regal dignity upon him.        --Sir T. More.
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            A law [of a State] which is a usurpation upon the
            general government.                   --O.
                                                  Ellsworth.
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            Manifest usurpation on the rights of other States.
                                                  --D. Webster.
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   Note: Usurpation, in a peculiar sense, formerly denoted the
         absolute ouster and dispossession of the patron of a
         church, by a stranger presenting a clerk to a vacant
         benefice, who us thereupon admitted and instituted.
         [1913 Webster]

   2. Use; usage; custom. [Obs.] --Bp. Pearson.
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WordNet (r) 3.0 (2006):

usurpation
    n 1: entry to another's property without right or permission
         [syn: trespass, encroachment, violation, intrusion,
         usurpation]
    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"