A measure of volatility of a public stock relative to an index or a composite of all stocks in a market or geographical region. A beta of more than one indicates the stock has higher volatility than the index (or composite) and a beta of one indicates volatility equivalent to the index (or composite). For example, the price of a stock with a beta of 1.5 will change by 1.5% if the index value changes by 1%. Typically, the S&P500 index is used in calculating the beta of a stock.