Beta

A measure of how much a share moves relative to the market. A beta of one moves with the index, above one amplifies it, below one dampens it.

It's estimated by regressing the stock's returns on the index's returns, usually over two to five years of weekly or monthly data.

Beta estimates are fragile in ways that matter. Different periods, frequencies and index choices produce materially different betas for the same company, and the figure for a thinly traded Indian mid-cap is often close to noise. So practitioners commonly use an industry average beta, unlevered and then relevered to the company's own capital structure, which is more stable and less specific.

A beta measures past co-movement, not future risk. The two are related, and they aren't the same.