Factorial

The factorial of a non-negative integer n, written n!, is the product of all positive integers from 1 up to n (and 0! is defined as 1). Factorials count the number of ways to arrange n distinct items and underpin permutations and combinations. They grow extremely fast — 10! is already 3,628,800 — so calculators switch to scientific notation for large values.

Example: 5! = 5 × 4 × 3 × 2 × 1 = 120.