Parkinson disease is a slowly progressive neurodegenerative disorder that occurs when certain nerve cells (neurons) in the midbrain area, called the substantia nigra, die or become impaired.

Normally, these neurons produce a vital substance known as dopamine. Dopamine is the chemical messenger responsible for transmitting signals between the substantia nigra (Latin for "black substance") and the next "relay station" of the brain, the corpus striatum, allowing smooth, coordinated function of the body’s muscles and movement.

The four cardinal features of Parkinson disease are tremor (shaking) at rest, bradykinesia (slowness of movement), rigidity (stiffness) of the limbs and trunk, and postural instability (impaired balance).

People who experience symptoms of Parkinson disease are generally referred to a neurologist. The disease is chronic, meaning it persists over a long period of time, and progressive, meaning that its symptoms gradually worsen over time. Parkinson disease is a very treatable neurologic disease. The goal of treatment is to alleviate symptoms and prevent orthopedic injury. Too much anti-Parkinson medication will result in more difficulties than the disease presents.

Parkinson disease is the most common form of parkinsonism, the label for a group of disorders with similar features. Parkinson disease is also called primary parkinsonism or idiopathic Parkinson disease; idiopathic is a term describing a disorder for which no cause has yet been found.