The Collaborative International Dictionary of English v.0.48:

Functional \Func"tion*al\ (f[u^][ng]k"sh[u^]n*al), a.
   1. Pertaining to, or connected with, a function or duty;
      official.
      [1913 Webster]

   2. (Pathology, Physiol.) Pertaining to the function of an
      organ or part, or to the functions in general; involving
      or affecting function rather than physiology; as,
      functional deafness; a functional disease. See functional
      disease, below. [WordNet sense 2]
      [1913 Webster]

   3. Designed for or capable of a particular function or use;
      as, a style of writing in which every word is functional;
      functional architecture. [WordNet sense 1]
      [WordNet 1.6]

   4. Fit or ready for use or service; useable; in working
      order; as, the toaster was still functional even after
      being dropped; the lawnmower is a bit rusty but still
      functional. Antonym of out of order and nonfunctional.
      [WordNet sense 4 & 6]

   Syn: usable, useable, in working order(predicate), operable,
        operational, running(prenominal), operative.
        [WordNet 1.6]

   5. Designed to emphasize practical utility rather than
      artistic or aesthetic purposes; as, functional education
      selects knowledge that is concrete and usable rather than
      abstract and theoretical; functional architecture; an
      amateurish device, crude but functional.
      [WordNet 1.6]

   Functional disease (Med.), a disease of which the symptoms
      cannot be referred to any appreciable lesion or change of
      structure; the derangement of an organ arising from a
      cause, often unknown, external to itself opposed to
      organic disease, in which the organ itself is affected.
      [1913 Webster]

WordNet (r) 3.0 (2006):

functional
    adj 1: designed for or capable of a particular function or use;
           "a style of writing in which every word is functional";
           "functional architecture" [ant: nonfunctional]
    2: involving or affecting function rather than physiology;
       "functional deafness" [ant: organic]
    3: relating to or based on function especially as opposed to
       structure; "the problem now is not a constitutional one; it
       is a functional one"; "delegates elected on a functional
       rather than a geographical basis"
    4: fit or ready for use or service; "the toaster was still
       functional even after being dropped"; "the lawnmower is a bit
       rusty but still usable"; "an operational aircraft"; "the
       dishwasher is now in working order" [syn: functional,
       usable, useable, operable, operational]
    5: designed for or adapted to a function or use; "functional
       education selects knowledge that is concrete and usable
       rather than abstract and theoretical"; "functional
       architecture"
    6: (of e.g. a machine) performing or capable of performing; "in
       running (or working) order"; "a functional set of brakes"
       [syn: running(a), operative, functional, working(a)]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "functional":
   acting, active, adjectival, advantageous, adverbial, agential,
   agentival, agentive, appropriate, at work, attributive, banausic,
   behavioral, beneficial, commodious, conjunctive, copulative,
   correct, effective, effectual, efficacious, efficient, employable,
   essential, expedient, fitting, formal, functioning, glossematic,
   go, going, going on, good for, grammatic, handy, helpful,
   important, in exercise, in force, in hand, in operation, in play,
   in practice, in process, in the works, inaction, industrial,
   intransitive, linking, managerial, manipulational,
   neuropsychiatric, nominal, of general utility, of help, of service,
   of use, official, on foot, on the fire, ongoing, operating,
   operational, operative, participial, performing, postpositional,
   practicable, practical, practicing, pragmatical, prepositional,
   pro, professional, pronominal, proper, psychiatric,
   psychobiological, psychodynamic, psychogenic, psychological,
   psychoneurological, psychopathic, psychopathological,
   psychophysical, psychophysiological, psychosexual, psychosocial,
   psychosomatic, psychotic, running, serviceable, serving,
   structural, substantive, syntactic, tagmemic, technical,
   transitive, useful, utilitarian, verbal, vocational, working



The Free On-line Dictionary of Computing (30 December 2018):

functional

   1. Working correctly.

   2. Pertaining to functional programming.

   3. higher-order function.