noun, plural hydras, hydrae [hahy-dree] , for 1–3, genitive hydrae [hahy-dree] , for 4.
(often initial capital letter) Classical Mythology. a water or marsh serpent with nine heads, each of which, if cut off, grew back as two; Hercules killed this serpent by cauterizing the necks as he cut off the heads.
any freshwater polyp of the genus Hydra and related genera, having a cylindrical body with a ring of tentacles surrounding the mouth, and usually living attached to rocks, plants, etc., but also capable of detaching and floating in the water.
a persistent or many-sided problem that presents new obstacles as soon as one aspect is solved.
(initial capital letter) Astronomy. the Sea Serpent, a large southern constellation extending through 90° of the sky, being the longest of all constellations.

1325–75; < Latin < Greek hýdrā water serpent (replacing Middle English ydre < Middle French < L); see otter

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World English Dictionary
hydra (ˈhaɪdrə)
n , pl -dras, -drae
1.  any solitary freshwater hydroid coelenterate of the genus Hydra, in which the body is a slender polyp with tentacles around the mouth
2.  a persistent trouble or evil: the hydra of the Irish problem
[C16: from Latin, from Greek hudra water serpent; compare otter]

Hydra1 (ˈhaɪdrə)
Greek myth a monster with nine heads, each of which, when struck off, was replaced by two new ones

Hydra2 (ˈhaɪdrə)
n , Latin genitive Hydrae
a very long faint constellation lying mainly in the S hemisphere and extending from near Virgo to Cancer

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1835, genus name of a freshwater polyp, from Gk. Hydra, many-headed Lernaean water serpent slain by Hercules (this sense is attested in Eng. from late 14c.), from hydor (gen. hydatos) "water" (see water (n.1)); related to Skt. udrah "aquatic animal" and O.E. ottur "otter."
Used figuratively for "any multiplicity of evils" [Johnson].
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
hydra   (hī'drə)  Pronunciation Key 
Plural hydras or hydrae (hī'drē)
See under hydroid.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences for Hydra
It was later revealed that the creature originated from the hydra system.
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