(1) A measurement of stock price volatility relative to a broad market index. If a stock moves up and down twice as much as the market, it has a beta of 2. If it moves one half as much as the market, its beta is 0.5. Because beta assumes a linear relationship, it can be seriously misleading if used in stock option evaluation or comparison. (2) The slope of a regression line. (3) The second tier of stocks listed on the London Stock Exchange. The other tiers are alpha (a) (4), delta (D) and gamma (g). See also alpha (a), delta (D) and gamma (g).