Definition:Variance/Discrete/Definition 3

Definition
Let $X$ be a discrete random variable.

Let $E \left({X}\right)$ be the expectation of $X$.

Then the variance of $X$, written $\operatorname{var} \left({X}\right)$, is defined as:
 * $\operatorname{var} \left({X}\right) := E \left({X^2}\right) - \left({E \left({X}\right)}\right)^2$

Also see

 * Variance as Expectation of Square minus Square of Expectation