The S&P 500, or the Standard & Poor's 500, is a large-cap American stock market index of 500 top-performing companies listing on the NYSE or NASDAQ. First released in 1957, the S&P 500 is a leading indicator of the performance of the U.S. market. The diversity of its companies makes the S&P 500 a popular tool for economic outlook; the index represents about 80% of available market capitalisation. There are a number of products based on the S&P 500 are available, but individual investors in the indices is uncommon because of its number of companies. The S&P 500 is free-float capitalisation-weighted.
Retrieved 04 Feb 2015 https://www.spglobal.com/spdji/en/indices/equity/sp-500/