Answer
$5!$ is called "5 factorial" and is the product of all positive integers from 5 down through 1. By definition, $0!$ = 1.
Work Step by Step
Taking n as a positive integer, the notation $n!$ (read "n factorial”) is defined as the product of all positive integers from n down through 1. We can write this as $n!=n(n-1)(n-2)(n-3).......(3)(2)(1)$. Now for $n=5$,
$5!=5\times 4\times 3\times 2\times 1$
And,
$\begin{align}
& n!=n(n-1)! \\
& 1!=1(0)! \\
& 0!=1
\end{align}$
