Standard Deviation

From Reuters Financial Glossary

The standard deviation (http://en.wikipedia.org/wiki/Standard_deviation) is a statistical measure of the amount by which one value in a range of values is likely to vary from the mean value. In finance it is used as a method of assessing how variable or volatile a price is likely to be. The bigger the standard deviation the more widespread are the price movements and the more volatile the performance of the stock or financial instrument.