Definition:Order of Group Element/Definition 1

Definition
Let $G$ be a group whose identity is $e_G$.

Let $x \in G$ be an element of $G$.

The order of $x$ (in $G$), denoted $\left\vert{x}\right\vert$, is the smallest $k \in \Z_{> 0}$ such that $x^k = e_G$.

Also see

 * Equivalence of Definitions of Order of Group Element