Source : WordNet®

standard deviation
     n : the square root of the variance

Source : Free On-Line Dictionary of Computing

standard deviation
     
         (SD) A measure of the range of values in a set of
        numbers.  Standard deviation is a statistic used as a measure
        of the dispersion or variation in a distribution, equal to the
        square root of the {arithmetic mean} of the squares of the
        deviations from the arithmetic mean.
     
        The standard deviation of a random variable or list of numbers
        (the lowercase greek sigma) is the square of the {variance}.
        The standard deviation of the list x1, x2, x3...xn is given by
        the formula:
     
         sigma = sqrt(((x1-(avg(x)))^2 + (x1-(avg(x)))^2 +
                      ...  + (xn(avg(x)))^2)/n)
     
        The formula is used when all of the values in the population
        are known.  If the values x1...xn are a random sample chosen
        from the population, then the sample Standard Deviation is
        calculated with same formula, except that (n-1) is used as the
        {denominator}.
     
        [{dictionary.com (http://dictionary.com/)}].
     
        ["Barrons Dictionary of Mathematical Terms, second edition"].
     
        (2003-05-06)