[ hahy-druh ]
/ ˈhaɪ drə /
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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.
(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.
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Origin of hydra

First recorded in 1325–75; from Latin hydra, from Greek hýdrā “water serpent” (replacing Middle English ydre, from Middle French, from Latin); see otter
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2021

How to use hydra in a sentence

British Dictionary definitions for hydra (1 of 3)

/ (ˈhaɪdrə) /

noun plural -dras or -drae (-driː)
any solitary freshwater hydroid coelenterate of the genus Hydra, in which the body is a slender polyp with tentacles around the mouth
a persistent trouble or evilthe hydra of the Irish problem

Word Origin for hydra

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

British Dictionary definitions for hydra (2 of 3)

/ (ˈhaɪdrə) /

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

British Dictionary definitions for hydra (3 of 3)

/ (ˈhaɪdrə) /

noun Latin genitive Hydrae (ˈhaɪdriː)
a very long faint constellation lying mainly in the S hemisphere and extending from near Virgo to Cancer
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

Scientific definitions for hydra

[ hīdrə ]

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