From The Collaborative International Dictionary of English v.0.48 :
Preemptive \Pre*["e]mp"tive\, a.
Of or pertaining to pre["e]mption; having power to
pre["e]mpt; pre["e]mpting.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
pre-emptive
adj 1: designed or having the power to deter or prevent an
anticipated situation or occurrence; "a preemptive
business offer" [syn: preemptive, pre-emptive]