Hydra

UK: 'Hydra', 'hydra': /ˈhaɪdrə/ US: (hī′drə)

Inflections of 'hydra' (n):

hydras

npl

hydrae

npl

Collins Concise English Dictionary © HarperCollins Publishers::

Hydra / ˈhaɪdrə/ noun

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

WordReference Random House Unabridged Dictionary of American English © 2025

hy•dra  (hī′drə), n., pl. -dras, -drae  (-drē) for 1–3,[gen.] -drae  (-drē) for 4.

1. Mythology (often cap.) [Class. Myth.] 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. Invertebrates 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. (cap.) [Astron.] the Sea Serpent, a large southern constellation extending through 90° of the sky, being the longest of all constellations.

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

Collins Concise English Dictionary © HarperCollins Publishers::

hydra / ˈhaɪdrə/ noun ( -dras, -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 evil

Etymology: 16th Century: from Latin, from Greek hudra water serpent; compare otter