U.S. policy that allowed the first settlers, or squatters, on public land to buy the land they had improved. Since improved land, coveted by speculators, was often priced too high for squatters to buy at auction, temporary preemptive laws allowed them to acquire it without bidding. The Pre-Emption Act (1841) gave squatters the right to buy 160 acres at $1.25 per acre before the land was auctioned. The Homestead Act (1862) made preemption an accepted part of U.S. land policy. Seealso Homestead Movement.

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• Preempt, a type of bid and a bidding tactic in contract bridge
• Preemption (computing), the ability of an operating system to stop a currently scheduled task in favour of a higher priority task
• Preemption (law), the displacing effect that US federal law has on a conflicting or inconsistent state law
• A "preemption" is also a form of broadcasting blackout
• Preemption is a type of land transfer in the United States, as in the Preemption Act of 1841
• Preemption Line, the line that divided the Indian lands of western New York State, that had been awarded to New York from those that had been awarded to the Commonwealth of Massachusetts by the Treaty of Hartford of 1786
• Pre-emption rights, the right of existing shareholders in a company to buy shares offered for sale before they are offered to the public
• Preemption (Media), the replacement of all or part of a regularly scheduled broadcast with a television special or breaking news.
• FDA Preemption, the legal theory in the United States that exempts product manufacturers from tort claims regarding Food and Drug Administration approved products

Other definitions

• Preemptive war