Definition

Factorial is the result of multiplying a sequence of descending natural numbers (such as 4 × 3 × 2 × 1)

The Factorial symbol is "!".

factorial symbol

4! is usually pronounced "4 factorial", but some people even say "4 shriek" or "4 bang".

Example

4! = 4 × 3 × 2 × 1 = 24
7! = 7 × 6 × 5 × 4 × 3 × 2 × 1 = 5040
1! = 1

Calculating From the Previous Value

We can easily calculate a factorial from the previous one:

nn!  
1111
22 × 1= 2 × 1!= 2
33 × 2 × 1= 3 × 2!= 6
44 × 3 × 2 × 1= 4 × 3!= 24
55 × 4 × 3 × 2 × 1= 5 × 4!= 120
6etc.etc.etc.

Reference & Resources

  • http://www.mathsisfun.com/numbers/factorial.html