standard deviation

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A statistic used as a measure of the dispersion or variation in a distribution or set of data, equal to the square root of the arithmetic mean of the squares of the deviations from the arithmetic mean.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun statistics A measure of how spread out data values are around the mean, defined as the square root of the variance.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the square root of the variance