Measure of variation shown by a set of observations
Sum of the squares of deviations from the mean divided by the number of observations minus 1 (because it is a sample)
$s^2=( \sum\nolimits_{i=1}^n (x_i- \mu)^2)/(n-1)$
Example:

Set of numbers: 3,4,5,5,5,6,7