Definition:Factorial/Definition 2

Definition
Let $n \in \Z_{\ge 0}$ be a positive integer.

The factorial of $n$ is defined as:
 * $n! = \displaystyle \prod_{k \mathop = 1}^n k = 1 \times 2 \times \cdots \times \left({n-1}\right) \times n$

where $\prod$ denotes product notation.

Also see

 * Equivalence of Definitions of Factorial