/ ˈhaɪdrə; NAmE / noun
1.
Hydra (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.
( 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 tube-shaped body and tentacles around its mouth
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WORD ORIGIN
via Latin from Greek hudra water snake, named by Linnaeus because, if cut into pieces, each section can grow into a whole animal.