A Psychosomatic disorder is a "condition in which psychological stresses adversely affect physiological (somatic) functioning to the point of distress". (Encyclopaedia Britannica).
"Psyche" means mind and "soma" means body. Therefore a physical or bodily manifestation of an underlying psychological condition is a psychological disorder. Note that we tend to use the word "Disorder" and not "disease". This is probably an inheritance in medicine from the old days when psychological conditions were not recognised as diseases and therefore what appeared to be a disease was called psychosomatic disorder if we suspected that the aetiology (underlying cause) was psychological. Today we are more ready to accept psychological conditions as illnesses and diseases.
One should not confuse this with the mental aspects of a physical disease, i.e. most disease will also cause a mental manifestations. In this situation the psychological or mental aspect is merely one of the end results of a disease process.
Some of the results of psychological state are obvious. An anxious person has a fast heart rate (tachycardia), he/she sweats and gets palpitations. However these are merely physical manifestations of a psychological state and not usually referred to as a disorder. On the other hand, a chronic anxiety or a repetitive stress causing a stomach ulcer is definitely a psychosomatic disorder, We may choose to work it slightly differently and say that the person has an ulcer disease, aggravated by chronic anxiety.
Other diseases that can be aggravated by psychological stress are , psoriasis, eczema, high blood pressure, and heart disease
Some of the true psychological disorders are simple thing like hives which comes up during psychological stress (histamine reaction) or blood pressure going up unduly when in a doctor's office.
Stress remains one of the biggest cause of many psychosomatic disorders or of aggravating many physical diseases.