WordNet (r) 3.0 (2006):

standard deviation
    n 1: the square root of the variance

The Free On-line Dictionary of Computing (19 January 2023):

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)