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hydra

[hahy-druh]
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noun, plural hy·dras, hy·drae [hahy-dree] /ˈhaɪ dri/ for 1–3, genitive hy·drae [hahy-dree] /ˈhaɪ dri/ for 4.
  1. (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.
  2. 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.
  3. a persistent or many-sided problem that presents new obstacles as soon as one aspect is solved.
  4. (initial capital letter) Astronomy. the Sea Serpent, a large southern constellation extending through 90° of the sky, being the longest of all constellations.
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Origin of hydra

1325–75; < Latin < Greek hýdrā water serpent (replacing Middle English ydre < Middle French < L); see otter
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for hydra

hydra

noun plural -dras or -drae (-driː)
  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 evilthe hydra of the Irish problem
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Word Origin

C16: from Latin, from Greek hudra water serpent; compare otter

Hydra1

noun
  1. Greek myth a monster with nine heads, each of which, when struck off, was replaced by two new ones
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Hydra2

noun Latin genitive Hydrae (ˈhaɪdriː)
  1. a very long faint constellation lying mainly in the S hemisphere and extending from near Virgo to Cancer
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for hydra

n.

1835, genus name of a freshwater polyp, from Greek Hydra, many-headed Lernaean water serpent slain by Hercules (this sense is attested in English from late 14c.), from hydor (genitive hydatos) "water" (see water (n.1)); related to Sanskrit udrah "aquatic animal" and Old English ottur "otter." Used figuratively for "any multiplicity of evils" [Johnson]. The fabulous beast's heads were said to grown back double when cut off, and the sea creature is said to be so called for its regenerative capabilities.

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Online Etymology Dictionary, © 2010 Douglas Harper

hydra in Science

hydra

[hīdrə]
Plural hydras hydrae (drē)
  1. See under hydroid.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.