Any law restricting business practices that are considered unfair or monopolistic.
Among U.S. laws, the best known is the Sherman Antitrust Act of 1890, which declared illegal "every contract, combination...or conspiracy in restraint of trade or commerce." The Clayton Antitrust Act of 1914, as amended in 1936 by the Robinson-Patman Act, prohibits discrimination among customers through prices or other means; it also prohibits mergers or acquisitions whenever the effect may be "to substantially lessen competition." Labour unions are also subject to antitrust laws.