Ataxia describes the inability of a patient to control muscle movement. It is typically tested by having the patient attempt to follow the doctor's finger or a similar object with the eyes while not moving the head, and then by having the patient attempt to grasp an object in front of them. It differs from convulsions as the muscles still respond to voluntary control and are still if no attempt is made to move them. It is apparently caused when muscles continue to fire even when the brain attempts to stop moving the muscle.
Among the disorders that most clearly demonstrate ataxia as a symptom is Parkinson's disease, where the muscles develop tremors whenever an attempt is made to move them. However, ataxia may have many causes such as brain damage, a response to a toxin or nerve damage.