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