If $\overline{x}$ is the mean of $n$ observations $x_1,x_2,\dots ,x_n$ then the mean of the absolute deviations of these observations from $\overline{x}$ is the mean deviation of the data.

Here,

$\overline{x}=\frac{x_1+x_2+...+x_n}{n} ....\left(1\right)$

Mean deviation

$=\frac{\left|x_1-\overline{x}\right|+\left|x_2-\overline{x}\right|+...+\left|x_n-\overline{x}\right|}{n}$