1. Pertaining to, or connected with, a function or duty; official.
2. (Science: physiology) Pertaining to the function of an organ or part, or to the functions in general.
(Science: medicine) functional disease, 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.