What s the difference between monopoly and oligony?

The definition of monopoly is one firm in the marketplace selling a particular good. An oligopoly is when a small group of firms comprise the market for a particular good. In the real world, there may be several, or even many, smaller competitors to a monopoly or an oligopoly, but the monopolist or the oligopoly still controls the vast share of the market. For example, Standard Oil repeatedly drove new entrants out of the market before its breakup.