- 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.
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- 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(biology) an extremely small water creature with a body shaped like a tube and tentacles around its mouth
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