preemption (Noun) — The right to purchase something in advance of others.
preemption (Noun) — The right of a government to seize or appropriate something (as property).
preemption (Noun) — A prior appropriation of something. ex. "the preemption of bandwidth by commercial interests"
preemption (Noun) — (law) the judicial principle asserting the supremacy of federal over state legislation on the same subject.