The S&P 500 is the U.S. stock-market index that tracks 500 large companies listed on American exchanges.
Standard & Poor’s introduced the modern S&P 500 in 1957. It is a float-adjusted, market-capitalization-weighted index, so companies with larger publicly tradable values generally have greater influence on its movements. The index includes companies from many industries rather than representing only one sector.
The S&P 500 is often used as a broad gauge of large-cap U.S. equities. It is not the same as the Dow Jones Industrial Average, which contains 30 companies and uses a price-weighting method. It is also different from the Nasdaq Composite, which covers thousands of Nasdaq-listed stocks.
Although its name says 500, the index can contain more than 500 individual stocks because some companies issue multiple eligible share classes. Index membership is determined by an S&P committee using published eligibility criteria.