The S&P 500 is a stock market index that measures the stock performance of 500 large companies listed on stock exchanges in the United States. It is widely regarded as one of the best representations of the U.S. stock market and a key indicator of the overall health of the economy. Investors and analysts use the S&P 500 to gauge market trends and make informed investment decisions. The index includes companies from various sectors, such as technology, healthcare, and finance, providing a comprehensive view of the market's performance.