What term describes a situation where a single seller controls trade in a good or service without close substitutes?

The term that describes a situation where a single seller controls trade in a good or service without close substitutes is monopoly. In a monopoly, one company or entity has significant market power and is the sole provider of a particular product or service, making it able to influence prices and supply levels without facing competition from other businesses. This typically happens when barriers to entry are high, preventing other firms from entering the market and competing.

In contrast, an oligopoly consists of a few firms that dominate a market, typically leading to interdependent decision-making regarding pricing and output. Monopolistic competition features many sellers offering differentiated products, which means consumers have options although sellers have some control over their pricing. Pure competition involves many sellers providing identical goods or services, resulting in no single seller having any influence over market prices. The distinct characteristics of a monopoly highlight the absence of competition and the unique market control that one seller possesses.