A form of imperfect competition where all firms simultaneously decide how much to produce. If there are only two firms, it is known as Cournot duopoly. Cournot competition produces an equilibrium with price greater than the competitive price and lower than the monopoly price. The converse holds for the total output. A Cournot equilibrium becomes more similar to the competitive equilibrium the more firms there are, and the better substitutes their products are for one another.
Reference: Oxford Press Dictonary of Economics, 5th edt.