Standard deviation is an index of the average deviation of a set of scores from the center of the distribution.
In statistics, standard deviation is a measure of the dispersion or variability of the scores in a distribution. It is the square root of the mean of the squares of each deviation from the mean. Or, to put it simply, the average distance of each score from the mean. Stanford-Bine! The American revision of the Binet scale, originally done at Stanford University in 1916 and now the most commonly used individual intelligence test in the English language.