Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) essential tremor ?
    ### Response:
    Essential tremor is a movement disorder that causes involuntary, rhythmic shaking (tremor), especially in the hands. It is distinguished from tremor that results from other disorders or known causes, such as Parkinson disease or head trauma. Essential tremor usually occurs alone, without other neurological signs or symptoms.
    