Antitrust refers to a series of laws enacted in the US and more recently in EEC to outlaw trusts and related monoplies. The purpose is to encourage fair competition for the benefit of consumers. Competition results in better quality, better service, and lower prices. The laws are administered by the Federal Trade Commission, who especially reviews mergers and acquisitions for potential to restrict competition.
In the US the basic law is the Sherman Antitrust Act enacted in 1890 in response to the numerous business cartels then popular. The best known was John D. Rockefeller's Standard Oil Trust which controlled the market for kerosene used in oil lamps. Standard Oil was broken up by the US Supreme Court in 1911.