Definitions
1.nounA 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.
2.nounA scalar-valued linear function on a vector space.
3.nounAn object encapsulating a function pointer (or equivalent).
4.adjectiveIn good working order.
5.adjectiveUseful; serving a purpose, fulfilling a function"That sculpture is not merely artistic, but also functional: it can be used as a hatrack."
6.adjectiveOnly for functional purposes, notably in architecture"A functional construction element generally must meet higher technical but lower aesthetical requirements"
7.adjectiveHaving semantics defined purely in terms of mathematical functions, without side-effects.
8.adjectiveOf a disease, such that its symptoms cannot be referred to any appreciable lesion or change of structure; opposed to organic disease, in which the organ itself is affected.