Source : Webster's Revised Unabridged Dictionary (1913)

Functional \Func"tion*al\, a.
   1. Pertaining to, or connected with, a function or duty;
      official.

   2. (Physiol.) Pertaining to the function of an organ or part,
      or to the functions in general.

   {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.