Definition of "functional"

In good working order.  examples

Useful; serving a purpose, fulfilling a function  examples

Only for functional purposes, notably in architecture  examples

(computing theory) Having semantics defined purely in terms of mathematical functions, without side-effects.  examples

(medicine) Of 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.  examples

(mathematics) A function that takes a function as its argument; more precisely, a function whose argument varies in a space of (real or complex valued) functions and whose value belongs to a monodimensional space.  examples

(mathematics, functional analysis) A scalar-valued linear function on a vector space.  examples

(computing) An object encapsulating a function pointer (or equivalent).  examples