economists define as a market structure with a few large firms
Market structure 2. Oligopoly, in which a market is run by a small number of firms that together control the majority of the market share.
Competition (economics) Later microeconomic theory distinguished between perfect competition and imperfect competition, concluding that perfect competition is Pareto efficient while imperfect competition is not.[citation needed] Competition, according to the theory, causes commercial firms to develop new products, services and technologies, which would give consumers greater selection and better products. The greater selection typically causes lower prices for the products, compared to what the price would be if there was no competition (monopoly) or little competition (oligopoly).