A market structure with few sellers, who realise their interdepend- ence in taking strategic decisions, for instance on price, output and quality. In an oligopoly, each firm is aware that its market behaviour will distinctly affect the other sellers and their market behaviour. As a result, each firm will take the possible reactions from the other players expressly into account. In competition cases, the term is often also used for situations where a few big sellers jointly dominate the competitive structure and a fringe of smaller sellers adapt to their behaviour. The big sellers are then referred to as the oligopolists. In certain circumstances this situation may be considered as one of collective (also joint or oligopolistic) dominance.
Source: Glossary of terms used in EU competition policy, Antitrust and control of concentrations, European Commission, 2002