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]