the.com/factorial

means A mathematical operation, written with an exclamation mark (n!), that multiplies a whole number by every positive whole number below it — so 5! means 5×4×3×2×1.

from From the noun 'factor' (a number you multiply by), itself from the Latin 'factor,' meaning 'maker' or 'doer,' from 'facere,' 'to make or do' — the same root that gives us 'factory' and 'manufacture.' The mathematical sense, with the tidy exclamation mark, was introduced in the early 19th century by the French mathematician Christian Kramp, who wanted a compact way to write these runaway products — and the punctuation feels apt for how fast the numbers explode.