Psychosomatic means a mind-body illness; an illness where emotional factors produce physical symptoms.
These arise mainly from overactivity of one of the parts of the nervous system called the autonomic nervous system, which is influenced by the emotional state of the individual.
For instance, chronic blushing may be due to feelings of guilt, and the mechanism of this condition is that small blood vessels in the face dilate as a result of a nervous overactivity, an inflammation follows, and the death of some cells results in the development of a rash.
The same mechanism of a blushing can occur also in the bowel; here the pathologic process is the same, resulting in ulcerative colitis, that is a serious inflammatory disease of the bowel.
Other pathologic conditions that are thought to be psychosomatic include asthma, migraine, urticaria, hay fever, peptic ulcer, hyperthyroidism, and several skin conditions. (Not yet proved!)
IMPORTANT: That a disorder is partly or wholly psychosomatic does not mean that it is all imagination; it is a real illness and can even sometimes be very serious.