In maths, the factorial of a whole number is what you get when you multiply that number by every whole number below it, down to 1. People write it with a tall bang: 5! means 5 × 4 × 3 × 2 × 1, which is 120. You meet this when you count how many ways you can line things up. It is a classroom and textbook word, not a street word. Though actually, 0! is a special case: it is defined as 1, which surprises a lot of students. The idea is simple, but the numbers get huge very fast — 10! is already 3,628,800. There is also an adjective factorial meaning “about a factor”, but the sense you need here is this multiplying one.