Economic Definition of HHI. Defined.
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Term HHI Definition: The common abbreviation for the Herfindahl-Hirshman index (or the Herfindahl index), which is a measure of concentration of the production in an industry that's calculated as the sum of the squares of market shares for each firm. This is an alternative method of summarizing the degree to which an industry is oligopolistic and the relative concentration of market power held by the largest firms in the industry. The Herfindahl index gives a better indication of the relative market control of the largest firms than can be found with the four-firm and eight-firm concentration ratios.