Definition of hydra

• (Greek mythology) monster with nine heads; when struck off each head was replaced by two new ones; "Hydra was slain by Hercules"
• a long faint constellation in the southern hemisphere near the equator stretching between Virgo and Cancer
• 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"