A stock index is a measurement that reflects the performance of a specific group of stocks, representing a segment of the financial market. 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 often used as benchmarks to compare the performance of individual stocks or investment portfolios. They can also serve as the basis for financial products like index funds and exchange-traded funds (ETFs), allowing investors to invest in a diversified portfolio that mirrors the index's performance.