Recursion is when something repeats. It does it over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over again.
That picture is having a smaller version of itself and a smaller and a smaller all the way down forever. It's recursion!
Actually, recursion also includes a situation where B follows A and A follows B right back. Also, recursion doesn't have to be infinite. So there you go, that's your geek lesson for today.
A vicious circle uses recursion. Recursion exemplifies a vicious circle.
Recursion is not in common usage as far as I know. The whole entry is a joke (duh...)