A stock index is a measurement that reflects the performance of a specific group of stocks, representing a segment of the stock market. It helps investors gauge market trends and the overall health of the economy. Common examples of stock indices include the S&P 500, which tracks 500 large companies in the U.S., and the Dow Jones Industrial Average, which includes 30 major U.S. companies. Investors use stock indices to compare the performance of individual stocks or investment portfolios. By analyzing these indices, they can make informed decisions about buying or selling stocks, as well as understanding market movements and economic conditions.