What does beta (β) mean?

Beta is a measure of how much an individual asset's price rises and falls in proportion to movements in the market as a whole, indicating systematic (market) risk that cannot be eliminated by diversification.

A beta greater than 1 is interpreted as tending to move more than the market, and a beta less than 1 as tending to move less. Beta does not measure the risk specific to an individual company, and because it is calculated from past data, there is no guarantee that it will hold in the future.