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."