Definition of hydra noun from the Oxford Advanced American Dictionary

     

    hydra

     noun
    noun
    NAmE//ˈhaɪdrə//
     
     
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  1. 1Hydra (in ancient Greek stories) a snake with several heads. As one head was cut off, another one grew. In the end it was killed by Hercules.
  2. 2(formal) a thing that is very difficult to deal with, because it continues for a long time or because it has many different aspects
  3. 3(biology) an extremely small water creature with a body shaped like a tube and tentacles around its mouth
See the Oxford Advanced Learner's Dictionary entry: hydra