What is Standard Deviation?
A statistical measure of how much returns spread around their average.
Formal Definition
Standard deviation quantifies the dispersion of a set of returns around their mean; in finance it is the standard measure of total volatility. A larger standard deviation means returns are spread more widely, implying greater uncertainty. It is the denominator in the Sharpe ratio and the basis for many risk models, including Value at Risk.
In Simple Terms
It is a number that says how spread out a stock's results usually are from its typical result. A small number means steady and predictable; a large number means all over the place.
Example
If a stock averages a 1% daily return with a standard deviation of 2%, most days fall roughly between minus 1% and plus 3%.