[Hy.dra] n [ME Ydra, fr. L Hydra, fr. Gk] 1: a many-headed serpent or monster in Greek mythology that was slain by Hercules and each head of which when cut off was replaced by two others
2. not cap: a multifarious evil not to be overcome by a single effort 3 [L (gen. Hydrae), fr. Gk]: a southern constellation of great length that lies south of Cancer, Sextans, Corvus, and Virgo and is represented on old maps by a serpent
4. not cap [NL, fr. L, Hydra]: any of numerous small tubular freshwater hydrozoan polyps (Hydra and related genera) having at one end a mouth surrounded by tentacles