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Definition of biological clock noun from the Oxford Advanced American Dictionary

     

    biological clock

     noun
    noun
     
    (technology)
     
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  1. 1a natural system in living things that controls regular physical activities such as sleeping
  2. 2(figurative) At 38, Kate's biological clock was ticking (= she was beginning to think that she would soon be too old to have children). The movements of the leaves are triggered by the plant's biological clock.
See the Oxford Advanced Learner's Dictionary entry: biological clock