One definition:

n! = 1 * 2 * 3 * ... * (n-1) * n

Another definition:

• 0! = 1
• n! = (n-1)! * n

• Factorial and Gamma function
• Factorial--from Eric Weisstein's World of Mathematics
• Pascal's Triangle, Factorials, Binomial Coefficient - Math Tables, Facts and Formulas - Hoxie High School Mathematics Department