Recursion means “defining something in terms of itself” usually at some smaller scale, perhaps multiple times, to achieve your objective. For example, we might say “A human being is someone whose mother is a human being”, or “a directory is a structure that holds files and (smaller) directories”, or “a family tree starts with a couple who have children, each with their own family sub-trees”.
Programming languages generally support recursion, which means that, in order to solve a problem, functions can call themselves to solve smaller subproblems.
Learning With Python LWP 