Pronunciation: (hī'dru),[key] —n., —pl.-dras, -draePronunciation: (-drē)[key]for 1–3, —gen.-draePronunciation: (-drē)[key]for 4. 1. (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. 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.