Monopoly is a market situation in U.S. Antitrust law where a single firm achieves an overwhelming dominance in a particular market such that it can exclude new market entrants. By this definition, when a company possesses 70% of the market, a monopoly market can be declared to exist; what matters is the ability to exclude new entrants. Contrary to a widespread impression, mere possession of a monopoly market is not illegal; for example, copyrights and patents are state-guaranteed monopolies of limited duration. Governments sometimes determine that the public’s best interest is served by declaring a legal monopoly, which is then carefully regulated to prevent abuses. See antitrust.