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.
      [1913 Webster]

            A law [of a State] which is a usurpation upon the
            general government.                   --O.
                                                  Ellsworth.
      [1913 Webster]

            Manifest usurpation on the rights of other States.
                                                  --D. Webster.
      [1913 Webster]

   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.
      [1913 Webster]

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"

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "usurpation":
   accession, adoption, anointing, anointment, appointment,
   appropriation, arrogation, assignment, assumption, authorization,
   colonization, conquest, consecration, coronation, delegation,
   deputation, election, empowerment, encroachment, enslavement,
   incursion, indent, infraction, infringement, inroad, intrusion,
   invasion, legitimate succession, occupation, overstepping,
   playing God, preemption, preoccupation, prepossession, requisition,
   seizure, subjugation, succession, takeover, taking over,
   transgression, trespass, trespassing



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

USURPATION, torts. The unlawful assumption of the use of property which
belongs to another; an interruption or the disturbing a man in his right and
possession. Toml. Law Dict. h.t.
     2. According to Lord Coke, there are two kinds of usurpation. 1. When a
stranger, without right, presents to a church, and his clerk is admitted;
and, 2. When a subject uses a franchise of the king without lawful
authority. Co. Litt. 277 b.



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

USURPATION, government. The tyrannical assumption of the government by force
contrary to and in violation of the constitution of the country.