Beta
A measure of how much an asset's returns move with the market, where 1.0 means it moves in line with the index.
Beta measures the sensitivity of an asset's returns to a benchmark, usually a broad market index. A beta of 1.0 means the asset has historically moved about as much as the index; 1.5 means about 50 percent more; 0.5 means about half as much. A negative beta means it has tended to move opposite the index.
On our site, beta is shown against a stated benchmark and window. Beta changes over time and depends on the period used, so treat it as an estimate.
Traders care because beta shows how much of a position's risk is market risk. A high-beta book adds to market exposure even when the individual stocks look different.
Worked example (illustrative numbers, not real data): a stock with a beta of 1.5 sits in a 100,000 dollar position. If the index falls 4 percent, the expected move is about 1.5 times 4 percent, or 6 percent, a loss of roughly 6,000 dollars before stock-specific effects.