factorial
Mathematical term for the product of all positive integers up to a given number. Used in combinatorics and probability.
The factorial of 5 is 120.
階乗の計算は、順列や組み合わせでよく使われる。
Factorial calculations are often used in permutations and combinations.
From 階 (かい, 'step' or 'rank') and 乗 (じょう, 'multiplication'), referring to the stepwise multiplication of integers.