A movement disorder characterized by involuntary, rapid, and wide movements, usually affecting the limbs. These movements are often irregular, violent, and occur on one side of the body (unilateral ballism), but can sometimes affect both sides (bilateral ballism). Ballysm often occurs when the subthalamic region, a structure in the basal ganglia responsible for regulating motor activity, is damaged. One common form is hemiballism, which affects only one side of the body.