- Hydra - (Greek mythology) monster with nine heads; when struck off each head was replaced by two new ones; "Hydra was slain by Hercules"
--1 is a kind of mythical monster, mythical creature
- Hydra, Snake - a long faint constellation in the southern hemisphere near the equator stretching between Virgo and Cancer
--2 is a kind of constellation
- hydra - trouble that cannot be overcome by a single effort because of its many aspects or its persistent and pervasive quality; "we may be facing a hydra that defies any easy solution"
--3 is a kind of trouble, problem
- hydra - small tubular solitary freshwater hydrozoan polyp
--4 is a kind of hydrozoan, hydroid
--4 is a member of genus Hydra - Hydra, Snake - a long faint constellation in the southern hemisphere near the equator stretching between Virgo and Cancer