A monopoly is a market condition in which a single seller controls the entire output of a particular good or service. A firm is a monopoly if it is the sole seller of its product and if its product has no close substitutes. Close substitutes are those goods that could closely take the place of a particular good; for example, a Pepsi soft drink would be a close substitute for a Coke drink, but a juice drink would not. The fundamental cause of monopoly is barriers to entry; these are technological or economic conditions of a market that raise the cost for firms wanting to enter the market above the cost for firms already in the market or otherwise make new entry difficult. If the barriers to entry prevent others firms from entering the market, there is no competition and the monopoly remains the only seller in its...