What is the value of 0! (zero factorial)?

The factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n. For example: 5! = 5 × 4 × 3 × 2 × 1 = 120. The value of 0! is 1, according to the convention for an empty product. The notation n! was introduced by the French mathematician Christian Kramp in 1808.