Definition:Variance/Discrete/Definition 3

Definition
Let $X$ be a discrete random variable.

Let $\expect X$ be the expectation of $X$.

Then the variance of $X$, written $\var X$, is defined as:
 * $\var X := \expect {X^2} - \paren {\expect X}^2$

Also see

 * Variance as Expectation of Square minus Square of Expectation