What is it called when there is only one buyer of a particular good or service?

The term that refers to a market condition where there is only one buyer of a particular good or service is called a monopsony. In this scenario, the singular buyer holds significant power over the supply chain and can influence the price and availability of the goods or services. This dynamic contrasts with a monopoly, where there is only one seller dominating the market, thereby allowing that seller to control the price of goods and services.

In a monopsony situation, the buyer’s power can lead to lower prices for the products or services they purchase, which may affect the suppliers, often resulting in reduced profit margins for them. Understanding monopsony is crucial because it highlights the buyer's market power and its implications for suppliers and competition within that market.

Other terms related to market structures, like duopoly (which involves two sellers), explore different competitive conditions, but they don't reflect the unique scenario of having a single buyer. Polypoly, although not commonly used in economic discourse, would refer to a market with many buyers and sellers. Thus, the specific nature of a monopsony makes it the correct answer in this context.