Definition of "Hydra" :

noun: small tubular solitary freshwater hydrozoan polyp

noun: 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."

noun: a long faint constellation in the southern hemisphere near the equator stretching between Virgo and Cancer

noun: (Greek mythology) monster with nine heads; when struck off each head was replaced by two new ones

"Hydra was slain by Hercules."