Definition of "Hydra"

(Greek mythology) A mythological serpent with many heads, slain by Hercules as one of his twelve labours.

(astronomy) A spring constellation of the northern sky, said to resemble a serpent. It lies just south of the zodiac and contains the star Alphard.  examples

(astronomy) One of Pluto's moons.  examples

One of the Saronic Islands in the Aegean Sea.  examples

(geography) A commune in Algiers Province, Algeria.  examples