Hydra is a genus of small freshwater hydrozoans of the phylum Cnidaria). They are native to the temperate and tropical regions). Hydra has a tubular, radially symmetric body up to 10 mm long when extended, secured by a simple adhesive foot known as the basal disc). They have two main body layers, which makes it "diploblastic". The layers are separated by mesoglea, a gel-like substance. The outer layer is the epidermis, and the inner layer is called the gastrodermis, because it lines the stomach). Hydra is characterized by its tentacles around the mouth opening. These tentacles can greatly extend to catch the prey or use for locomotion. Each tentacle is clothed with many stinging cells. Upon contact with prey, the contents of the stinging cells are explosively discharged, firing stings that can paralyze small animals. Biologists are especially interested in Hydra because of their regenerative ability; they do not appear to die of old age, or to age at all). Hydra are generally sedentary or sessile, but do occasionally move quite readily, especially when hunting. They have two distinct methods for moving – looping and somersaulting).

In Greek mythology, Hydra was a poisonous, many-headed serpent with frustratingly effective powers of regeneration – cut off one of its heads and two would grow in its place. The destruction of the Lernean Hydra became one of the 12 Labours of Heracles. In modern English, hydra or hydra-headed can describe a difficult or multifarious situation.

It is important to note that HYDRA is also an authoritarian-subversive paramilitary terrorist organization in the Marvel Cinematic Universe.