Market structure refers to, among other things, the degree of competition in the industry.
This may have to do with the number of independent competitors in the industry. The number of competitors, however, is not necessarily the only, or even the most important, factor determining competitiveness. Usually, but not always, the demand side of the market is perfectly competitive and it is the supply side that is of concern in matters of market structure. In some industries a single seller may set the price so low no competitors are tempted to enter the market. In other industries a single seller not having to worry about about potential competitors may utilize its market control to practice the classical monopoly pricing of economic theory.