Standard & Poor's 500 Index

Standard & Poor's 500 Index (S&P 500). The benchmark Standard & Poor's 500 Index, widely referred to as the S&P 500, tracks the performance of 500 widely held large-cap US stocks in the industrial, transportation, utility, and financial sectors.

In calculating the changing value of this capitalization-weighted index, also called a market value index, stocks with the greatest number of floating shares trading at the highest share prices are weighted more heavily than stocks with lower market value.

This can mean that a relatively few stocks have a major impact on the movement of the index at any point in time. The stocks included in the index, their relative weightings, and the number of stocks from each of the sectors vary from time to time, at S&P's discretion.