- 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.
- 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.
- a persistent or many-sided problem that presents new obstacles as soon as one aspect is solved.
- the Sea Serpent, a large southern constellation extending through 90° of the sky, being the longest of all constellations.
- any solitary freshwater hydroid coelenterate of the genus Hydra, in which the body is a slender polyp with tentacles around the mouth
- a persistent trouble or evil
- a monster with nine heads, each of which, when struck off, was replaced by two new ones
- a very long faint constellation lying mainly in the S hemisphere and extending from near Virgo to Cancer
