The factorial of a number $n$ (denoted as $n!$) is the product of all positive integers from 1 to $n$. The base case for recursion is $0!=1$.

Code Example:

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

print(factorial(5))  # Output: 120

This function calculates the factorial of a number using recursion.