Variance measures how much the values in a data set deviate from the mean. It is a key concept in probability and statistics that quantifies data dispersion.
\[ \sigma^2 = s^2 = \frac{\sum (x_i - \bar{x})^2}{n - 1} = \frac{\sum x_i^2 - \left(\sum x_i\right)^2/n}{n - 1} \]