Source : Webster's Revised Unabridged Dictionary (1913)

Preemption \Pre*["e]mp"tion\ (?; 215), n. [Pref. pre- + emption:
   cf. F. pr['e]emption. See {Redeem}.]
   The act or right of purchasing before others. Specifically:
   (a) The privilege or prerogative formerly enjoyed by the king
       of buying provisions for his household in preference to
       others. [Eng.]
   (b) The right of an actual settler upon public lands
       (particularly those of the United States) to purchase a
       certain portion at a fixed price in preference to all
       other applicants. --Abbott.

Source : WordNet®

preemption
     n 1: the judicial principle asserting the supremacy of federal
          over state legislation on the same subject [syn: {pre-emption}]
     2: the right of a government to seize or appropriate something
        (as property) [syn: {pre-emption}]
     3: the right to purchase something in advance of others [syn: {pre-emption}]
     4: a prior appropriation of something; "the preemption of
        bandwidth by commercial interests" [syn: {pre-emption}]