A monopoly is characterized by a single seller that produces a unique product with few substitutes and operates in a market with high barriers to entry. Monopolies may earn long-run profits by producing less output than would exist under perfect competition and charging higher prices. While this leads to deadweight loss, monopolies can persist due to barriers like control of resources, economies of scale, or patents. Governments address monopolies through policies like breaking up firms, reducing trade barriers, price regulation, or natural monopoly regulation.