Final answer:
The four-firm concentration ratio measures the percentage of total industry sales accounted for by the four largest firms, which helps to understand the extent of monopoly power and competitiveness in a market.
Step-by-step explanation:
The concept being asked about pertains to the four-firm concentration ratio, which is a measure of market concentration within an industry. This ratio indicates the percentage of total sales that the four largest firms in an industry account for. As a tool used by regulators to gauge the degree of monopoly power, a high concentration ratio can signify a market with less competition and potential inefficiencies, akin to a monopoly situation.