The S&P 500, or Standard & Poor's 500, is a stock market index that measures the performance of 500 of the largest publicly traded companies in the United States. It is widely regarded as a key indicator of the overall health of the U.S. economy and the stock market. The index includes companies from various sectors, such as technology, healthcare, and finance, providing a broad representation of the market. Investors and analysts often use the S&P 500 to gauge market trends and make investment decisions. The index is weighted by market capitalization, meaning larger companies have a greater impact on its performance. It is commonly used as a benchmark for investment portfolios and mutual funds.