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]