Beta
According to asset pricing theory, beta represents the type of risk, systematic risk, that cannot be diversified away. By definition, the market itself has a beta of 1.0, and individual stocks are ranked according to how much they deviate from the macro market. A stock with a beta of 2 has returns that change, on average, by twice the magnitude of the overall market's returns; when the market's return falls or rises by 3%, the stock's return will fall or rise (respectively) by 6% on average.