wiki:https://en.wikipedia.org/wiki/Factorial

In mathematics, the factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n. For example,

5 ! = 5 × 4 × 3 × 2 × 1 = 120.   {\displaystyle 5!=5\times 4\times 3\times 2\times 1=120.\ }

The value of 0! is 1, according to the convention for an empty product.