A stock market index is a measurement that reflects the performance of a specific group of stocks. It helps investors gauge the overall health of the stock market or a particular sector. Common indices include the S&P 500, which tracks 500 large companies in the U.S., and the Dow Jones Industrial Average, which includes 30 significant U.S. companies. Indices are calculated using various methods, such as price-weighted or market capitalization-weighted approaches. They serve as benchmarks for investors to compare the performance of their investments against the broader market. Additionally, many financial products, like index funds, are designed to replicate the performance of these indices.