A numerical measure of the way the price of a representative group of Stocks has changed over time. Every major exchange has one or more indices. For example, the NASDAQ exchange has the NASDAQ 100 Index (a composite of the100 largest non-financial companies listed on NASDAQ). Some indices are created and managed by private corporations, such as the Dow Jones Industrial Average and S&P 500. You can trade CFDs based on many of the world's stock market indices.