Recursion is a programming and mathematical concept where a function calls itself to solve a problem. It breaks down complex problems into smaller, more manageable sub-problems. Each time the function calls itself, it works on a simpler version of the original problem until it reaches a base case, which is a condition that stops the recursion. A common example of recursion is calculating the factorial of a number, denoted as n!. The factorial of a number n is the product of all positive integers up to n. For instance, 5! equals 5 × 4 × 3 × 2 × 1, and this can be computed using a recursive function that calls itself with decreasing values of n.