A psychosomatic response was a medical condition that was caused by an incorrect belief or stress.
In 2372, Doctor Julian Bashir attributed Elim Garak's nosebleed to a psychosomatic response. He also listed several known psychosomatic responses, such as changes in vital signs and spontaneous tumor growth. When Worf asked if further more serious injuries were possible, Bashir replied that a broken bone was unlikely, but stoppage of his heart or bursting a blood vessel in his brain was possible. (DS9: "Things Past")