The Collaborative International Dictionary of English v.0.48:

Hydra \Hy"dra\, n.; pl. E. Hydras, L. Hydr[ae]. [L. hydra,
   Gr. "y`dra; akin to "y`dwr water. See Otter the animal,
   Water.]
   1. (Class. Myth.) A serpent or monster in the lake or marsh
      of Lerna, in the Peloponnesus, represented as having many
      heads, one of which, when cut off, was immediately
      succeeded by two others, unless the wound was cauterized.
      It was slain by Hercules. Hence, a terrible monster.
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            Gorgons, and Hydras, and Chimeras dire. --Milton.
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   2. Hence: A multifarious evil, or an evil having many
      sources; not to be overcome by a single effort.
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   3. (Zool.) Any small fresh-water hydroid of the genus
      Hydra, usually found attached to sticks, stones, etc.,
      by a basal sucker.
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   Note: The body is a simple tube, having a mouth at one
         extremity, surrounded by a circle of tentacles with
         which it captures its prey. Young hydras bud out from
         the sides of the older ones, but soon become detached
         and are then like their parent. Hydras are remarkable
         for their power of repairing injuries; for if the body
         be divided in pieces, each piece will grow into a
         complete hydra, to which fact the name alludes. The
         zooids or hydranths of marine hydroids are sometimes
         called hydras.
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   4. (Astron.) A southern constellation of great length lying
      southerly from Cancer, Leo, and Virgo.
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