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
Cite this Entry
“Hydra.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/Hydra. Accessed 31 Oct. 2025.
Copy CitationMedical Definition
Geographical Definition
Hydra
geographical name
Hy·dra
ˈhī-drə 
variants
or Modern Greek Ídhra
island of Greece in the southern Aegean Sea off the east coast of the Peloponnese area 20 square miles (52 square kilometers), population 2794
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