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The percentage of an industry or sector that a single company controls. For example, if Retail Company A conducts 10% of all retail sales in the United States, it is said to have a 10% market share or penetration. It is important that a company, especially a large company, maintains a substantial market share in order to remain competitive. However, companies that achieve too high a market share through mergers, acquisitions, or other methods may become regarded as monopolies and violate local antitrust laws. Some companies have a large market share compared to their competitors, but not enough to be considered a monopoly: these are called gorillas. See also: Herfindahl-Hirschman Index.