For the best definition of Functional Medicine, I quote from the Textbook of Functional Medicine:

“Functional Medicine is a dynamic approach to assessing, preventing, and treating complex chronic disease. 

Functional  Medicine helps clinicians identify and ameliorate dysfunctions in the human body as a method of improving patient health. Functional Medicine acknowledges that chronic disease is almost always preceded by a lengthy period of declining function in one or more of the body’s systems. Returning patients to health requires reversing (or substantially improving) the specific dysfunctions that have contributed to the disease state. 

Those dysfunctions are, for each of us, the result of lifelong interactions among our environment, our lifestyle, and our genetic predispositions.  Each patient, therefore represents a unique, complex, and interwoven set of influences on intrinsic functionality that have set the stage for disease or the maintenance of health.“

David Jones, MD. The Textbook of Functional Medicine, Gig Harbor, WA