An abbreviation for the Standard & Poor’s 500 Index. It is a free-float weighted measurement stock market index of the 500 largest companies listed on stock exchanges in the United States. It is one of the most commonly followed equity indices. It is widely regarded as the best gauge of large-cap US equities. Institutional investors consider the S&P 500 as more representative of US equity markets than the Dow Jones due to the S&P comprising more stock across all sectors (500 vs the 30 of the Dow).