Final answer:
A monopoly is a market structure in which there is only one producer/seller for a product. Entry into such a market is restricted due to high costs or other barriers.
Step-by-step explanation:
A market structure characterized by only one producer/seller for a product is called a monopoly. In a monopoly, there are no close competitors, and the single business has control over the product's price and supply. Entry into a monopoly market is restricted due to high costs or other barriers, such as legal or technological obstacles.