The Collaborative International Dictionary of English v.0.48:

Usurp \U*surp"\, v. t. [imp. & p. p. Usurped; p. pr. & vb. n.
   Usurping.] [L. usurpare, usurpatum, to make use of, enjoy,
   get possession of, usurp; the first part of usurpare is akin
   to usus use (see Use, n.): cf. F. usurper.]
   To seize, and hold in possession, by force, or without right;
   as, to usurp a throne; to usurp the prerogatives of the
   crown; to usurp power; to usurp the right of a patron is to
   oust or dispossess him.
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         Alack, thou dost usurp authority.        --Shak.
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         Another revolution, to get rid of this illegitimate and
         usurped government, would of course be perfectly
         justifiable.                             --Burke.
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   Note: Usurp is applied to seizure and use of office,
         functions, powers, rights, etc.; it is not applied to
         common dispossession of private property.
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   Syn: To arrogate; assume; appropriate.
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The Collaborative International Dictionary of English v.0.48:

Usurp \U*surp"\, v. i.
   To commit forcible seizure of place, power, functions, or the
   like, without right; to commit unjust encroachments; to be,
   or act as, a usurper.
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         The parish churches on which the Presbyterians and
         fanatics had usurped.                    --Evelyn.
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         And now the Spirits of the Mind
         Are busy with poor Peter Bell;
         Upon the rights of visual sense
         Usurping, with a prevalence
         More terrible than magic spell.          --Wordsworth.
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WordNet (r) 3.0 (2006):

usurp
    v 1: seize and take control without authority and possibly with
         force; take as one's right or possession; "He assumed to
         himself the right to fill all positions in the town"; "he
         usurped my rights"; "She seized control of the throne after
         her husband died" [syn: assume, usurp, seize, take
         over, arrogate]
    2: take the place of; "gloom had usurped mirth at the party
       after the news of the terrorist act broke"

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "usurp":
   accroach, adopt, advance upon, appropriate, arrogate, assume,
   assume command, break bounds, colonize, commandeer, conquer,
   cut out, displace, encroach, enslave, go too far, hog, indent,
   infringe, intrude, invade, irrupt, jump a claim, know no bounds,
   make an inroad, make free with, make use of, monopolize,
   mount the throne, occupy, overrun, overstep, overstep the bounds,
   play God, preempt, preoccupy, prepossess, pretend to, requisition,
   seize, seize power, seize the throne, sit on, squat on, subjugate,
   take all of, take charge, take command, take it all, take over,
   take possession of, take the helm, take the lead, take up,
   transgress, trespass