Factorial

Factorial comes from factor, which you have learned about. (Have a look back at that lesson if you could do with a reminder.) The end part of the word Factorial, the -ial part means having the character of. So factorial means having the character of, or being like, factors.

The definition of factorial is the product of a positive integer multiplied by all smaller integers. For example four factorial is

The symbol for factorial is the explanation mark (!). We write four factorial as 4!