The factorial n! is defined for a positive integer as the product of the same number by all numbers preceding up to 1:

n!=nx(n-1)x(n-2)x...x3x2x1

For example the factorial of 5 is:

5!= 5x4x3x2x1 =120

PROPERTIES

It is also possible to define the factorial in recursive way, defining the value of 1!  so that you can calculate the next factorial one:

1!=1

n!=nx(n-1)!

It turns out that:
2!=2x1!=2x1=2
3!=3x2!=3x2=6
4!=4x3!=4x6=24
and so on.