antitrust The government restriction of monopolies. Violations of antitrust laws are enforced by the Antitrust Division of the U.S. Department of Justice. Antitrust legislation in the United States originated with the Sherman Antitrust Act of 1890. This act made collusion in the market illegal. The Clayton Antitrust Act of 1914, along with the Robinson-Patman Act of 1936, furthered antitrust legislation by targeting mergers and acquisitions that could lead to monopolies.