functional

adj

1. In good working order.

   “While we assume that phonological subsyllabic units are functional prior to reading skill, we have not ruled out a possible interaction with the writing system.”

2. Useful; serving a purpose, fulfilling a function.

   “That sculpture is not merely artistic, but also functional: it can be used as a hatrack.”

   “The two are often one and the same, says Walt Thompson, professor of kinesiology at Georgia State University and lead author of the ACSM survey – although functional fitness can apply to the younger generation as well.”

3. Only for functional purposes, notably in architecture.

   “A functional construction element generally must meet higher technical but lower aesthetical requirements.”

4. Of or relating to one's role or office; official.

   “functional responsibilities”

5. Characterizing functioning in environment, being symptoms that do not presuppose alteration of organic structure.
6. Optimized for all-around physiological benefit.

   “functional coffee”

   “functional fitness”

7. Of or relating to a function or functions.
8. Having semantics defined purely in terms of mathematical functions, without side-effects.

noun

1. 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.

   “An example of a functional is the definite integration of integrable real functions in a real interval.”

2. A scalar-valued linear function on a vector space.
3. An object encapsulating a function pointer (or equivalent).