Parkinson's disease is a chronic progressive nervous disease chiefly of later life marked by tremor and weakness of resting muscles and by a shuffling gait. A disorder of the brain, it can result in difficulty with walking, movement, and coordination.From answers.com: Parkinson's disease (PD) is a progressive movement disorder marked by tremors, rigidity, slow movements (bradykinesia), and posture instability. It occurs when cells in one of the movement-control centers of the brain begin to die for unknown reasons. PD was first noted by British physician James Parkinson in the early 1800s.