Beta
Principles of Finance glossary · Risk, Return & Portfolio Theory
Definition
The sensitivity of an asset's return to the return on the market portfolio; a measure of systematic risk.
Beta in plain English
Beta answers one question: when the whole stock market lurches, how much does THIS stock lurch along with it? A beta of 1 means the stock tends to move step-for-step with the market; a beta of 2 means it swings about twice as hard (great on the way up, brutal on the way down); a beta of 0.5 means it only half-reacts. Think of the market as the ocean tide and each stock as a boat — beta measures how tightly that boat rises and falls with the tide, versus bobbing on its own. This 'moves-with-everything' risk matters because it's the part you can't erase by owning lots of different stocks, so investors demand extra reward for carrying it.
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