An index is an indicator or measure of something, and in finance, it typically refers to a statistical measure of change in a securities market. In the case of financial markets, stock, and bond market indices consist of a hypothetical portfolio of securities representing a particular market or a segment of it. (You cannot invest directly in an index.) The S&P 500 and the US Aggregate Bond Index are common benchmarks for the American stock and bond markets, respectively. In reference to mortgages, it refers to a benchmark interest rate created by a third party—Read more at Investopedia. Chen, James. “Index.” 23 March 2019