A functional disorder is a medical condition that impairs normal functioning of bodily processes that remains largely undetected under examination, dissection or even under a microscope. At the exterior, there is no appearance of abnormality. This stands in contrast to a structural disorder (in which some part of the body can be seen to be abnormal) or a psychosomatic disorder (in which symptoms are caused by psychological or psychiatric illness). Definitions vary somewhat between fields of medicine.
Generally, the mechanism that causes a functional disorder is unknown, poorly understood, or occasionally unimportant for treatment purposes. The brain or nerves are often believed to be involved. It is common that a person with one functional disorder will have others.