Factorial of n, written n!, is the product of all positive integers from 1 to n.
5! = 5 × 4 × 3 × 2 × 1 = 120
Special cases: 0! = 1 (by definition, this makes formulas work), 1! = 1.
Factorials grow fast. 10! = 3,628,800. This is the number of ways to arrange 10 distinct items.