Changes

A monopoly ''(from the Greek words "Mono," meaning "Singular" and "Poly," meaning "plentiful")'' is a single firm that sells a product having no close substitutes. The firm has almost no competitors.

A monopoly ''(from the Greek words "Mono," meaning "Singular" and "Poly," meaning "plentiful")'' is a single firm that effectively has no competitors in its market. The monopoly may then charge consumers a price that exceeds its marginal cost, but only in situations where the monopoly good does not have close substitutes. Monopolies producing goods with close substitutes may be monopolies in their own narrow market, but would not be able to enjoy the market power typically associated with monopolies.

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

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