Parkinson's disease: A slowly progressive neurologic disease that is characterized by a fixed inexpressive face, tremor at rest, slowing of voluntary movements, gait with short accelerating steps, peculiar posture and muscle weakness (caused by degeneration of an area of the brain called the basal ganglia), and low production of the neurotransmitter dopamine. Most patients are over 50, but at least 10 percent are under 40. Treatment involves use of medication, such as levodopa (brand name: Larodopa) and carbidopa (brand name: Sinemet). A surgical procedure known as deep brain stimulation, in which externally controlled electrodes are implanted into the brain, has also been shown to be helpful. There are no blood or laboratory tests to diagnose the condition. Although it is a chronic and progressive disease, the degree of disability varies among affected persons. Also known as paralysis agitans and shaking palsy.