The factorial of a number is defined as the product of all the numbers from 1 through that number.
For example:

• the factorial of 3 is defined as 1*2*3 (the value of which is 6)
• The factorial of 7 is defined as 1*2*3*4*5*6*7 (the value of which is 5040)

The factorial of 1 is defined to be 1
The factorial of 0 (zero) is also defined to be 1
The factorial of a negative number is undefined.

NOTE: In math, the shorthand notation 7! is used to represent the factorial of 7 (also pronounced as "seven factorial")