variance

The variance of a random variable is a measure of dispersion from the mean.

Formally,
the variance of a random variable $X$ is given by

Var (X) = E [(X - μ)^{2}]

where $E$ is the expected value and $μ$ is the mean $μ = E [X]$ .

This can be simplified to

Var (X) = E [X^{2}] - μ^{2}

The variance is the covariance of a random variable with itself.