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):

preempt
    n 1: a high bid that is intended to prevent the opposing players
         from bidding [syn: preemptive bid, pre-empt, preempt]
    v 1: acquire for oneself before others can do so
    2: take the place of or have precedence over; "live broadcast of
       the presidential debate preempts the regular news hour";
       "discussion of the emergency situation will preempt the
       lecture by the professor" [syn: preempt, displace]
    3: gain possession of by prior right or opportunity, especially
       so as to obtain the right to buy (land)
    4: make a preemptive bid in the game of bridge

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