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.
Microeconomics Microeconomics (from Greek prefix mikro- meaning "small" + economics) is a branch of economics that studies the behavior of individuals and firms in making decisions regarding the allocation of scarce resources and the interactions among these individuals and firms.[1][2][3]