herfindahl-hirschman index means: It is a measure of market concentration. This depends on how many firms there are and the size of the market they compete in. This is done by adding up squares of the market share of all firms. Concentrated markets are those where the HHI exceeds 1800. Antitrust concerns may be raised by the Department of Justice if mergers or acquisitions result in the reduction of the HHI of more than 100 percent. (in Stock Market Dictionary)