A market structure with one seller and high barriers preventing competing firms from entering is a monopoly.
A monopolist is the sole supplier of a particular good or service in a defined market. Because there are no close competitors, the firm faces the market demand curve and can influence price by choosing how much to produce. Barriers to entry may come from legal rights, control of an essential resource, large economies of scale, or network effects.
A monopoly is different from an oligopoly, where a small number of firms dominate the market. Perfect competition has many sellers offering highly similar products with easy entry and exit, while monopolistic competition has many sellers offering differentiated products.
Natural monopolies can arise when one firm can supply the market at lower average cost than multiple firms, as in some utility networks. Governments may regulate prices, operate the service, or use competition policy when monopoly power creates higher prices, reduced output, or weaker incentives to innovate.