Final answer:
A monopoly is differentiated from an oligopoly by the existence of a single producer with unique products and extremely high barriers to entry, leading to no competition and allowing for long-run economic profits. Oligopolies, however, consist of a few competing firms even though they also have high entry barriers.
Step-by-step explanation:
The characteristic that distinguishes a monopoly from an oligopoly is the presence of a single producer with a unique product and very high barriers to entry. In a monopoly, these barriers prevent other competitors from entering the market, allowing the monopolist to control the market price and earn significant long-run economic profits. By contrast, an oligopoly consists of a few large firms that dominate the market with similar products, and while there are also high barriers to entry, these firms can face competition from each other, making the market more competitive than a monopoly.
Oligopolies often exhibit product differentiation and interdependence among firms, changing output or prices in response to competitors' actions. Conversely, monopolies have no competitors within their market, controlling the quantity demanded and quantity supplied, which lets them manipulate prices due to the lack of close substitutes for their unique product. Therefore, while both market structures have barriers to entry, the lack of competition is a defining characteristic of a monopoly.