The Herfindahl-Hirschman Index (HHI) is a common measure of market concentration and is used to determine market competitiveness, often pre- and post-Merger & Acquisition transactions. 

• It is calculated by squaring the market share of each firm competing in a market and then summing the resulting numbers. It can range from close to zero to 10,000. 
• Regulators use the HHI Index using the 50 largest companies in a particular industry to determine if that industry should be considered competitive or as close to being a monopoly. 

Source: Investopedia