Emily Dickinson Lexicon

HY'DRA, n. [L. hydra; Gr. υδρα, from υδωρ, water.]

1. A water serpent. In fabulous history, a serpent or monster in the lake or marsh of Lerna, in Peloponnesus; represented as having many heads, one of which, being cut off, was immediately succeeded by another, unless the wound was cauterized. Hercules killed this monster by applying firebrands to the wounds, as he cut off the heads. Hence we give the name to a multitude of evils, or to a cause of multifarious evils.
2. A technical name of a genus of Zoophytes, popularly called polypus, or polypuses.
3. A southern constellation, containing sixty stars. Cyc.