This document defines oligopoly as a market structure with a few firms selling standardized or differentiated products. It has the following key characteristics: few firms with large sizes, homogeneous or differentiated products, mutual interdependence where firms consider rivals' reactions, and barriers to entry that restrict new competition. Firms in an oligopoly engage in non-price competition through advertising and innovation. They also face a kinked demand curve that creates price rigidity and a range where costs can change without affecting profits. Game theory can model the strategic interactions between oligopolistic rivals.