Definition:Variance/Discrete/Definition 1

Definition
Let $X$ be a discrete random variable.

Then the variance of $X$, written $\operatorname{var} \left({X}\right)$, is a measure of how much the values of $X$ varies from the expectation $E \left({X}\right)$, and is defined as:
 * $\operatorname{var} \left({X}\right) := E \left({\left({X - E \left({X}\right)}\right)^2}\right)$

That is: it is the expectation of the squares of the deviations from the expectation.

Also see

 * Equivalence of Definitions of Variance