Monopoly

A monopoly is a single firm that sells a product having no close substitutes. The firm has almost no competitors.

In the United States, as a result of the Sherman Antitrust Act of 1890 and subsequent legislation, monopolies are illegal; more specifically,

While it is not illegal to have a monopoly position in a market, the antitrust laws make it unlawful to maintain or attempt to create a monopoly through tactics that either unreasonably exclude firms from the market or significantly impair their ability to compete.