A degenerative nervous disorder occurring mostly in persons older than 60 years, characterized in advanced stages by rhythmic muscular tremors, especially in the hands, and by rigidity of movement, a drooping posture, slow speech, and a masklike facial expression; the term designates a set of symptoms, and the underlying disease causing these symptoms is most commonly Parkinson's disease. Parkinsonism results from a deficiency in the neurotransmitter dopamine caused by degeneration of the cells producing that agent. It can be mitigated by chemotherapy with agents such as levodopa (3-hydroxy-L-tyrosine).  [WordNet 1.5]