Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Tremor ?
    ### Response:
    Tremor is an unintentional, rhythmic, muscle movement involving to-and-fro movements of one or more parts of the body. Most tremors occur in the hands, although they can also affect the arms, head, face, voice, trunk, and legs. Sometimes tremor is a symptom of another neurological disorder or a side effect of certain drugs, but the most common form occurs in otherwise healthy people.
    