Hydra
noun
Hy·dra ˈhī-drə 
1
: 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
First Known Use
15th century, in the meaning defined at sense 1
Medical Definition
hydra
noun
hy·dra ˈhī-drə
: any of numerous small tubular freshwater hydrozoan polyps (Hydra and related genera) having at one end a mouth surrounded by tentacles
Geographical Definition
Hydra
geographical name
Hy·dra ˈhī-drə
variants or Modern Greek Ídhra
ˈēt͟h-rä
island of Greece in the southern Aegean Sea off the east coast of the Peloponnese area 20 square miles (52 square kilometers), population 2794
Hydriot
ˈhī-drē-ət
noun -drē-ˌät
or Hydriote
ˈhī-drē-ˌōt
-ət