A statistical measure of the relative volatility of a stock, fund, or other security in comparison with the market as a whole. The beta for the market is 1.00. Stocks with betas above 1.0 are more responsive to the market, but are also more risky investments. Stocks with a beta below 1.0 tend to move in the opposite direction of the market. For example, if the market moves 10%, a stock with a beta of 3.00 will move 30%; a stock with a beta of .5 will move 5%.