It means factorial of 5. The operation method is 5, 4, 3, 2 and 1, and the result is 120. Factorial factor is an operation symbol invented by Keyston Kaman in 1808, and it is a mathematical term. The factorial of a positive integer is the product of all positive integers less than or equal to this number, and the factorial of 0 is 1. In 1808, Kaman Keston introduced this notation. Because the factorial of a positive integer is a continuous multiplication operation, the result of multiplying 0 by any real number is 0. So we can't generalize or deduce the factorial of 0 with the definition of positive integer factorial.