What does x! mean?

for a postive integer x, x! is the product

 

x(x-1)(x-2)...(3)(2)(1)

 

for example 

 

1! = 1

 

2! = 2

 

3! = 6

...

6! = 6 x 5 x 4 x 3 x 2 = 720

 

0! = 1 is also used on occasion