A monopoly (from the Greekmonos, one + polein, to sell) is when a product or service can only be bought from one supplier for a specific market. If other firms serve the market, they may be too small compared to the largest, thus allowing the large firm to have a lot of market power. A monopoly usually happens when there is no economic competition to produce the product or service and there is no available substitutes to the product or service.
In economics, a monopoly is a single producer of a product or service. In law, a monopoly is a firm that has a lot of market power and is able to charge very high prices for a product or service. As long as the firm has a lot of market power, it does not matter if the firm is large or small, as size is not used to decide if a firm is a monopoly.
A monopoly can be formed by the government, by merging a few firms to form a large one, or form naturally to become a natural monopoly. In many places, utilities such as telephone service or cable television are monopolies, as it is very costly to build the infrastructure for another firm to compete in the market. Such natural monopolies is hence allowed by the government, but is usually strictly controlled to prevent the monopoly from charging a very high price for a product or service.
Many countries, including the United States, have laws to stop companies from having a monopoly or to limit the actions that they can do as a monopoly.