The Collaborative International Dictionary of English v.0.48:

Unseat \Un*seat"\, v. t. [1st pref. un- + seat.]
   [1913 Webster]
   1. To throw from one's seat; to deprive of a seat. --Cowper.
      [1913 Webster]

   2. Specifically, to deprive of the right to sit in a
      legislative body, as for fraud in election. --Macaulay.
      [1913 Webster]
      [1913 Webster]

WordNet (r) 3.0 (2006):

unseat
    v 1: remove from political office; "The Republicans are trying
         to unseat the liberal Democrat"
    2: dislodge from one's seat, as from a horse

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "unseat":
   break, buck off, bust, cashier, deconsecrate, defrock, depose,
   deracinate, dethrone, disarticulate, disbar, discrown, disenthrone,
   disjoint, dislocate, dislodge, dismiss, dismount, displace,
   drum out, excommunicate, expel, give the gate, kick upstairs,
   liquidate, luxate, oust, overthrow, pension, pension off, pitch,
   purge, read out of, remove, remove from office, retire, root out,
   root up, strip of office, strip of rank, superannuate, suspend,
   throw off, unchurch, unfrock, unhinge, unhorse, unjoint, unplace,
   unsaddle, unthrone, uproot