In Ms-Excel, there is no such function named, Function function. According to me, the question maybe Function within a function is correct. If it is the question, we call it as nesting functions. Nesting Function means a function within another function. That is, a function may be one of the argument of another function. Example: sum(average(1,2,3),7) returns the value 10. First the average function returns the value 3 which is an argument for sum function, adds it with 7 and returns the value 10.