In economic theory monopoly is a term used to describe a market in which there is only one seller of a good or service. This is obviously a case of dominance. The concept of dominance also extends to all those cases where one firm has a large share of the market. However, it is not possible to define precisely how large this share has to be. Dominance clearly exists when the leading firm has 80 per cent of the market, but what if it has 50 per cent? Much then depends on the size of the next largest firm. If the nearest rival has, say, 10 per cent of the market, most would agree that a measure of dominance exists, but if there are only two other firms with shares of 30 per cent and 20 per cent the position is less clear.