Beta

In finance, beta is used to measure a stock's volatility (how much it goes up and down) to that of the market. If the stock moves up or down by an average of 10% and the market does the same, then that stock has a beta of 1.

However, if a stock has twice the volatility (moves up or down twice as much) of the market, that stock has a beta of 2. A beta of 0.5 means that the stock is half as volatile as the market in question.