Inner Function: The inner function is the first or "inside" function in a composite function. It's applied to an input value before passing its output to another (outer) function.
Outer Function: The outer function is the second or "outside" layer of a composite function. It takes as its input (or argument) what comes out from applying an inner (first/inside) function on some initial value.
Function Composition: This term refers to combining two or more functions together in order to create a new composite one.