The S&P 500 is a benchmark stock market index that measures the performance of 500 of the largest publicly traded companies in the United States. Created in 1957 by Standard & Poor's, it covers a broad range of industries and is considered a key indicator of overall U.S. market health. Investors and analysts use the S&P 500 to assess economic trends, portfolio performance, and market sentiment, making it one of the most closely watched financial metrics worldwide.