Definition of functional:
in good working order; useful; serving a purpose
"that sculpture is not merely artistic, but also functional. It can be used as a hatrack."
a function that takes a function as its argument
; More precisely: A function y=f(x) whose argument x varies in a space of (real valued, complex valued) functions and whose value belongs to a monodimensional space. An example: the definite integration of integrable real functions in a real interval