Functional medicine is a field of medicine that employs assessment and intervention to improve physiological, emotional/cognitive, and physical function. In this context, function is the "kind of action or activity proper to a person, thing, or institution."(1) functional medicine vs. traditional medicine: what's the difference?

Functional medicine is science-based health care that demands a systematic, patient-centered approach to understanding the underlying web of physiological factors and effects that influence health and the progression of disease.

It incorporates the functional principles that exist in many conventional and alternative practices but focuses, as a special core competence, on the principles of molecular medicine and modern nutritional biochemistry with an emphasis on clinical application. 

1. The Random House Dictionary of the English Language. New York: Random House