A psychosomatic illness was one in which the patient manifested symptoms that were caused chiefly by the power of suggestion, not an actual infection or injury.
As of 2268, Lieutenant Mira Romaine's psychological profile recorded occasional instances of this during her adolescence that were not above average in occurrence. Doctor Leonard McCoy informed Captain James T. Kirk of this when the captain inquired about pertinent aspects of her profile relating to her symptoms after a visit to Memory Alpha. (TOS: "The Lights of Zetar")