Factorial is a mathematical function denoted as n!, where n is a non-negative integer. Definition: n! = 1·2·3·...·n 0! = 1

The factorial of a number n (denoted as n!) is the product of all positive integers from 1 to n inclusive. By definition, 0! = 1.

Example implementation of factorial calculation in Python using recursion:

def factorial(n):
    if n == 0:
        return 1
    else:
        return n * factorial(n - 1)

# Example usage
print(factorial(5))  # Outputs 120

It can also be implemented using a loop:

def factorial_iterative(n):
    result = 1
    for i in range(1, n + 1):
        result *= i
    return result

print(factorial_iterative(5))  # 120