in the United States, a stock market index that tracks 500 publicly traded domestic companies. It is considered by many investors to be the best overall measurement of American stock market performance.

Standard & Poor's, which sponsors a number of other market indexes, traces its roots to an investment information service begun in 1860 by Henry Varnum Poor. In 1941 Poor's original company, Poor's Publishing, merged with Standard Statistics (founded in 1906 as the Standard Statistics Bureau) and assumed the name Standard and Poor's Corporation, a provider of financial information and analysis. The S&P 500 index, formerly called the Composite Index (and later Standard & Poor's Composite Index), had been launched on a small scale in 1923. It began tracking 90 stocks in 1926 and expanded to 500 in 1957. Unlike the Dow Jones average, the S&P 500 computes a weighted average of the stocks constituting the index. As a result, the stocks with a larger market valuation have a greater impact on the overall index.

The companies listed on the S&P 500 represent a who's who of U.S. industry, and additions and deletions from the list often indicate market trends. Some of the top-weighted companies in the index include General Electric Company (General Electric Co.), Microsoft Corporation, Citigroup Inc., and Exxon Mobil Corporation. Holding companies and real-estate stocks are not eligible for the list. The company was acquired by the McGraw-Hill Companies, Inc., in 1966.