When a function (or procedure) calls itself. Such a function is called "recursive". If the call is via one or more other functions then this group of functions are called "mutually recursive".
If a function will always call itself, however it is called, then it will never terminate. Usually however, it first performs some test on its arguments to check for a "base case" - a condition under which it can return a value without calling itself.
The canonical example of a recursive function is factorial:
factorial 0 = 1 factorial n = n * factorial (n-1)
Functional programming languages rely heavily on recursion, using it where a procedural language would use iteration.
See also recursion, recursive definition, tail recursion.