What term describes a market structure where one company is the only seller of a product?

The term that describes a market structure where one company is the only seller of a product is "monopoly." In a monopoly, a single firm has significant control over the market, allowing it to dictate prices and supply without facing competition from other sellers. This structure can occur due to high barriers to entry, unique product offerings, or government regulation that grants exclusive rights to the single seller.

In contrast, an oligopoly consists of a small number of firms that dominate the market, leading to strategic interactions among them. A duopoly is a specific type of oligopoly where only two companies share the market. Perfect competition features many sellers providing identical products, allowing for free entry and exit in the market, which is quite the opposite of a monopoly's single-seller scenario. Thus, monopoly is the correct term for the described market structure.