A stock index is a measurement that reflects the performance of a group of stocks, representing a specific market or sector. Common examples include the S&P 500, which tracks 500 of the largest U.S. companies, and the Dow Jones Industrial Average, which includes 30 major U.S. corporations. Investors use stock indices to gauge market trends and make informed investment decisions. Stock indices are calculated using various methods, such as price-weighted or market capitalization-weighted approaches. They provide a snapshot of market performance, helping investors understand how well the overall market or specific sectors are doing over time.