The Collaborative International Dictionary of English v.0.48:

Preempt \Pre*["e]mpt"\ (?; 215), v. t. & i. [imp. & p. p.
   Pre["e]mpted; p. pr. & vb. n. Pre["e]mpting.] [See
   Pre["e]mption.]
   To settle upon (public land) with a right of preemption, as
   under the laws of the United States; to take by
   pre["e]mption. [Also spelled pre-empt.]
   [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

pre-empt
    n 1: a high bid that is intended to prevent the opposing players
         from bidding [syn: preemptive bid, pre-empt, preempt]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "preempt":
   accroach, adopt, annex, appropriate, arrogate, assume, colonize,
   commandeer, confiscate, conquer, enslave, expropriate, hog, indent,
   jump a claim, make free with, make use of, monopolize, occupy,
   overrun, preoccupy, prepossess, requisition, seize, sequester,
   sit on, squat on, subjugate, take, take all of, take it all,
   take over, take possession of, take up, usurp