In economics, a market structure with a single seller and no close substitute for its product is called a monopoly.
A monopolist supplies the market and faces the market demand curve. Unlike a firm in perfect competition, it is not simply a price taker. Its ability to influence price depends on demand, production costs, and barriers that prevent competitors from entering.
Barriers can include legal protections, control of an essential resource, large economies of scale, or network effects. A natural monopoly may arise when one provider can serve the market at lower average cost than multiple competing providers, as can happen in some infrastructure industries.
Monopoly does not mean that the seller can charge any price without consequences. Higher prices can reduce quantity demanded, and regulators may impose rules on prices or service. An oligopoly instead has a small number of significant sellers, while monopolistic competition has many sellers offering differentiated products.