Define the concept of recursion in programming languages.

Recursion in programming languages is a method where a function calls itself to solve a problem.

Recursion is a fundamental concept in computer science and programming. It is a technique where a function, instead of performing a task, divides the task into smaller subtasks and calls itself to solve these subtasks. This process continues until a base condition is met, which is a scenario where the function can solve the problem without further recursion.

To understand recursion, consider the example of calculating the factorial of a number. The factorial of a number n (denoted as n!) is the product of all positive integers less than or equal to n. This can be calculated using a recursive function. The function would first check if the number is 0 or 1 (the base case), in which case it would return 1. If not, it would return the product of the number and the factorial of the number minus one. This process continues until the base case is reached.

Recursion can be a powerful tool in programming, but it must be used with care. Each recursive call adds a layer to the system's call stack, which can lead to a stack overflow if the recursion is too deep. This is why it's crucial to always have a base case that can be reached in a reasonable number of steps.

Moreover, recursion can sometimes lead to inefficient solutions, especially if the same subproblem is solved multiple times. This can be mitigated by techniques such as memoisation, which involves storing the results of expensive function calls and reusing them when the same inputs occur again.

In conclusion, recursion is a key concept in programming that allows functions to call themselves to solve problems. It can be a powerful tool, but it must be used wisely to avoid potential pitfalls.

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