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.

Each index related to the stock and bond markets has its own calculation methodology. In most cases, the relative change of an index is more important than the actual numeric value representing the index. For example, if the Financial Times Stock Exchange (FTSE) 100 is at 6,670.40, that number tells investors the index is nearly seven times its base level of 1,000. However, to assess how the index has changed from the previous day, investors must look at the amount the index has fallen, often expressed as a percentage.