Recursion. BWAAAAM

• A method or function that calls itself.
• A type of iteration or looping.
• Must have a stopping condition.(To prevent endless loops)

Two types: 

• Tail: when the last thing that happens is the recursive call.
• Head: when it happens first.

Some situations in programming can only be solved with recursion. (e.g.: displaying a list of files within a directory structure)

Factorial

multiplication of every value between zero and itself.