Definition of 'Monopoly'. Tweet. A market structure characterized by a single seller, selling a unique product in the market. Definition: A market structure characterized by a single seller, selling a unique product in the market. In a monopoly market, the seller faces no competition, as he is the sole seller of goods with no close substitute. Description: In a monopoly market, factors like government license, ownership of resources, copyright and patent and high starting cost make an entity a single seller of goods.