1 Greek Mythology a many-headed snake whose heads grew again as they were cut off, killed by Hercules.
(as noun hydra
) a thing that is hard to overcome or resist because of its pervasive or enduring quality or its many aspects.
2 Astronomy the largest constellation (the Water Snake or Sea Monster), said to represent the beast slain by Hercules. Its few bright stars are close to the celestial equator. Compare with Hydrus.
(as genitive Hydrae
/ˈhīdrē/) used with a preceding letter or numeral to designate a star in this constellation:the star Beta Hydrae