<p><strong>Noun</strong></p><ul><li>Genitive: Hydrae [hahy-dree].</li><li>Plural: hydras, hydrae [hahy-dree]. Often Hydra.</li><li>Classical Mythology: 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.</li><li>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.</li><li>A persistent or many-sided problem that presents new obstacles as soon as one aspect is solved.</li><li>Genitive: Hydrae [hahy-dree].</li><li>(Initial capital letter) Astronomy: The Sea Serpent, a large southern constellation extending through 90° of the sky, being the longest of all constellations.</li></ul>