: a many-headed serpent or monster in Greek mythology that was slain by Hercules and each head of which when cut off was replaced by two others
2
not capitalized: a multifarious evil not to be overcome by a single effort
3
: a southern constellation of great length that lies south of Cancer, Sextans, Corvus, and Virgo and is represented on old maps by a serpent
4
not capitalized: any of numerous small tubular freshwater hydrozoan polyps (Hydra and related genera) having at one end a mouth surrounded by tentacles
Word History
Etymology
Middle English Ydra, Idre, borrowed from Latin Hydra "any of various mythical monsters, especially the Hydra of Lerna killed by Hercules, a constellation," borrowed from Greek hýdra "aquatic snake, the Hydra of Lerna, a constellation"; (sense 4) borrowed from New Latin, genus name, going back to Latin — more at otter