1. adjective
a) In good working order.
That sculpture is not merely artistic, but also functional: it can be used as a hatrack.
b) Useful; serving a purpose, fulfilling a function
A functional construction element generally must meet higher technical but lower aesthetical requirements
See Also: functionary
2. noun
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.
Wikipedia foundation.