What is Pre-emption?

Legal Definition
When state law and federal law conflict, federal law displaces, or preempts, state law, due to the Supremacy Clause of the Constitution. U.S. Const. art. VI., § 2. Preemption applies regardless of whether the conflicting laws come from legislatures, courts, administrative agencies, or constitutions. For example, the Voting Rights Act, an act of Congress, preempts state constitutions, and FDA regulations may preempt state court judgments in cases involving prescription drugs.

Congress has preempted state regulation in many areas. In some cases, such as medical devices, Congress preempted all state regulation. In others, such as labels on prescription drugs, Congress allowed federal regulatory agencies to set national minimum standards, but did not preempt state regulations imposing more stringent standards than those imposed by federal regulators. Where rules or regulations do not clearly state whether or not preemption should apply, the Supreme Court tries to follow lawmakers’ intent, and prefers interpretations that avoid preempting state laws.
Legal Definition
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; an 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.
-- Bouviers Law Dictionary
Legal Definition
In international law. The right of pre-emption 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. In English law. The first buying of a thing. A privilege formerly enjoyed by the crown, of buying up provisions and other necessaries, by the intervention of the king's purveyors, for the use of his royal household, at an appraised valuation, in preference to all others, and even without consent of the owner. 1 Bl. Comm. 287; Garcia v. Callender, 125 N. Y. 307, 26 N. E. 283. In the United States, the right of preemption is a privilege accorded by the government to the actual settler upon a certain limited portion of the public domain, to purchase such tract at a fixed price to the exclusion of all other applicants. Nix v. Allen, 112 U. S. 129, 5 Sup. Ch 70, 28 I Ed. 675; Bray v. Ragsdale, 53 Mo. 170.
See also
-- Black's Law Dictionary
Legal Definition
To set up a prior claim, or the right to gain an advantage before anybody else.
Legal Definition
A buying before another person.
-- Ballentine's Law Dictionary