| Noun | 1. | Hydra - (Greek mythology) monster with nine heads; when struck off each head was replaced by two new ones; "Hydra was slain by Hercules" | |
| 2. | Hydra - a long faint constellation in the southern hemisphere near the equator stretching between Virgo and Cancer | |
| 3. | 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"trouble, problem - a source of difficulty; "one trouble after another delayed the job"; "what's the problem?" | |
| 4. | hydra - small tubular solitary freshwater hydrozoan polyp | |