a measure of dispersion, computed by taking the arithmetic mean of the absolute values of the deviations of the functional values from some central value, usually the mean or median.
British Dictionary definitions for average-deviation
mean deviation
noun (statistics)
1.
the difference between an observed value of a variable and its mean
2.
Also mean deviation from the mean, mean deviation from the median, average deviation. a measure of dispersion derived by computing the mean of the absolute values of the differences between observed values of a variable and the variable's mean