What is the difference between an Oligopoly and a Monopoly?

A monopoly and an oligopoly are economic market structures where there is imperfect competition in the market. A monopoly market contains a single firm that produces goods with no close substitute, with significant barriers to entry of other firms. An oligopoly market has a small number of relatively large firms that produce similar but slightly different products. Again, there are significant barriers to entry for other enterprises.

The geographical size of the market can determine whether there is an oligopoly or a monopoly. A firm may dominate an industry in a particular area where there are no alternatives to the same product but have two or three similar companies operating nationwide. Thus, the firm may be a monopoly in a region but operate in an oligopoly market in a larger geographical area.