Definition of Hydra
- Hydra Proper noun A mythological serpent with seven heads, slain by Hercules as one of his twelve labours. Also called the Lernaean Hydra.
- Hydra Proper noun A spring constellation of the northern sky, said to resemble a serpent. It lies just south of the zodiac and contains the star Alphard.
- Hydra Proper noun The name of one of Pluto's moons. (Named 21 June 2006).