pre·emp·tion or pre-emp·tion   (prē-ěmp'shən)    n.   The right to purchase something before others, especially the right to purchase public land that is granted to one who has settled on that land. A purchase made by such a right. Prior seizure of, appropriation of, or claim to something, such as property. [pre- + Latin ēmptiō, ēmptiōn-, buying (from ēmptus, past participle of Latin emere, to buy; see em- in Indo-European roots).]

Main Entry: pre·emp·tion
Pronunciation: prE-'emp-sh&n
Function: noun
Etymology: Medieval Latin praeemption- praeemptio previous purchase, from praeemere to buy before
1 a : the right of purchasing before others : PREEMPTIVE RIGHT b : a right to purchase a tract of public land before others that was given by the government to the actual occupant of the land
NOTE: This sense of preemption is primarily of historical importance.
2 : a doctrine in conflicts of law: when a superior government (as of a state) has undertaken to regulate a subject its laws supersede those of an inferior government (as of a municipality)
NOTE: According to the doctrine of preemption, federal law supersedes state law when federal law is in conflict with a state law on a subject or when there is congressional intent to regulate a subject to the exclusion of the states. Federal preemption is based on the supremacy clause of the U.S. Constitution and is closely related to the powers granted Congress in the commerce clause.
3 : an act or instance of preempting