A functional disorder is a medical condition that impairs the normal function of a bodily process, but where every part of the body looks completely normal under examination, dissection or even under a microscope. 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.