The Collaborative International Dictionary of English v.0.48:

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.
       [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "preemption":
   adoption, appropriation, arrogation, assumption, colonization,
   conquest, emption, enslavement, first option, first refusal,
   indent, occupation, option, preoccupation, prepossession, refusal,
   requisition, right of emption, right of preemption, subjugation,
   takeover, taking over, usurpation



Bouvier's Law Dictionary, Revised 6th Ed (1856):

PRE-EMPTION, intern. law. The right of preemption is the right of a nation
to detain the merchandise of strangers passing through her territories or
seas, in order to afford to her subjects the preference of purchase. 1 Chit.
Com. Law, 103; 1 Bl. Com. 287.
     2. This right is sometimes regulated by treaty. In that which was made
between the United States and Great Britain, bearing date the 10th day of
November, 1794, ratified in 1795, it was agreed, art. 18, after mentioning
that the usual munitions of war, and also naval materials should be
confiscated as contraband, that "whereas the difficulty of agreeing on
precise cases in which alone provisions and other articles not generally
contraband may be regarded as such, renders it expedient to provide against
the inconveniences and misunderstandings which might thence arise. It is
further agreed that whenever any such articles so being contraband according
to the existing laws of nations, shall for that reason be seized, the same
shall not be confiscated, but the owners thereof shall be speedily and
completely indemnified; and the captors, or in their default, the government
under whose authority they act, shall pay to the masters or owners of such
vessel the full value of all articles, with a reasonable mercantile profit
thereon, together with the freight, and also the damages incident to such
detention." See Mann. Com. B. 3, c. 8.
     3. By the laws of the United States the right given to settlers of
public lands, to purchase them in preference to others, is called the
preemption right. See act of L. April 29, 1830, 4 Sharsw. Cont. of Story, U.
S. 2212.