Source : Webster's Revised Unabridged Dictionary (1913)

Preemptive \Pre*["e]mp"tive\, a.
   Of or pertaining to pre["e]mption; having power to
   pre["e]mpt; pre["e]mpting.

Source : WordNet®

preemptive
     adj : designed or having the power to deter or prevent an
           anticipated situation or occurrence; "a preemptive
           business offer"