What is a monopoly?

A monopoly is defined as a market structure where a single seller or producer has significant control over a particular market. This results in the seller being the sole provider of a good or service, which leads to a lack of competition. In a monopoly, the firm has the power to dictate prices, often leading to higher prices and reduced availability of products for consumers compared to competitive markets. The absence of rival firms means there are no alternatives for consumers, giving the monopolist substantial influence over the market.

The other contexts provided in the question highlight different types of market scenarios. In a market with multiple sellers, for instance, competition exists, which allows consumers to have choices and typically leads to better prices and innovation. Similarly, a structure characterized by significant competition is far removed from a monopoly since it entails numerous firms vying for consumer attention in a dynamic environment. Government regulation may impact market structures, but it is not a defining characteristic of what constitutes a monopoly. Thus, the essence of a monopoly lies in the dominance of a single seller in the market, making the provided answer accurate.