standard deviation

1 :  a measure of the dispersion of a frequency distribution that is the square root of the arithmetic mean of the squares of the deviation of each of the class frequencies from the arithmetic mean of the frequency distribution; also :  a similar quantity found by dividing by one less than the number of squares in the sum of squares instead of taking the arithmetic mean

2 :  a parameter that indicates the way in which a probability function or a probability density function is centered around its mean and that is equal to the square root of the moment in which the deviation from the mean is squared