1. A constellation located between Libra and Centaurus to the south, and Cancer to the north.

2. The outermost moon of the dwarf planet Pluto.

3. A genus of 31 species of freshwater animals (family Hydridae), found throughout temperate and tropical regions. They are about 10 mm long and resemble a tube with a foot at one end by which they anchor themselves and a mouth surrounded by up to 12 stinging tentacles at the other end, used to capture the small animals on which they feed. Most hydra die from disease, poisoning by contaminants in the water, or by being eaten, but those that survive show no sign of ageing and under ideal circumstances they may be immortal.