S&P 500
^GSPC

The S&P 500 is an index containing the stocks of 500 Large-Cap corporations, most of which are American. The index is the most notable of the many indices owned and maintained by Standard & Poor’s, a division of McGraw-Hill. S&P 500 is used in reference not only to the index but also to the 500 actual companies whose stocks are included in the index. The S&P 500 index forms part of the broader S&P 1500 and S&P Global 1200 stock market indices. All of the stocks in the index are those of large publicly held companies and trade on the two largest US stock markets, the New York Stock Exchange and Nasdaq. After the Dow Jones Industrial Average, the S&P 500 is the most widely watched index of large-cap US stocks. It is considered to be a bellwether for the US economy and is a component of the Index of Leading Indicators. It is often quoted using the symbol SPX, and may be prefixed with a caret (^) or with a dollar sign ($). Many index funds and exchange-traded funds track the performance of the S&P 500 by holding the same stocks as the index, in the same proportions, and thus attempting to match its performance (before fees and expenses). Partly because of this, a company which has its stock added to the list may see a boost in its stock price as the managers of the mutual funds must purchase that company’s stock in order to match the funds’ composition to that of the S&P 500 index. In stock and mutual fund performance charts, the S&P 500 index is often used as a baseline for comparison. The chart will show the S&P 500 index, with the performance of the target stock or fund overlaid. Prior to 1957, the primary S&P stock market index consisted of 90 companies, known as the S&P 90, and was published on a daily basis. A broader index of 423 companies was also published weekly. On March 4, 1957, a broad, real-time stock market index, the S&P 500 was introduced. This introduction was made possible by advancements in the computer industry which allowed the index to be calculated and disseminated in real time. The index was previously market-value weighted; that is, movements in price of companies whose total market valuation (share price times the number of outstanding shares) is larger will have a greater effect on the index than companies whose market valuation is smaller. The index has since been converted to float weighted; that is, only shares which Standard & Poors determines are available for public trading ("float") are counted. (From Wikipedia)