It's the term of art in political economy when only a few sellers exist in a market. The converse terms are, by the way monopsony (when there's only one buyer in the market) and oligopsony, when there are only a few buyers in the market. Amazon's the dominant player in an oligopolistic marketplace, since there aren't many online booksellers in a position to compete with it. On the other hand, the market for thermonuclear weapons is oligopsonistic, since there are remarkably few buyers.