Definition of Monopoly: 1  A pure monopoly is defined as a single seller of a product, i.e. 100% of market share. 2  In the UK a firm is said to have monopoly power if it has more than 25% of the market share. 3  For example, Tesco @30% market share or Google 90% of search engine traffic.efinition of Monopoly: 1  A pure monopoly is defined as a single seller of a product, i.e. 100% of market share. 2  In the UK a firm is said to have monopoly power if it has more than 25% of the market share. 3  For example, Tesco @30% market share or Google 90% of search engine traffic.