Factorial (noun)

In mathematics, the factorial of a positive integer is the product of all the positive integers up to and including that number.

Origin:

From latin 'factorium', from 'facere' (to do or to make).

Examples:

1. The factorial of 5 is equal to 5 x 4 x 3 x 2 x 1, which equals 120.
2. The factorial function is a basic operation in combinatorial mathematics.
3. In probability theory, factorials are used to calculate the number of permutations and combinations.
4. The factorial of 0 is defined to be 1.
5. The factorial notation is often represented by an exclamation mark, as in 5! = 120.