Hydra (wrestler) definition

(1) Small tubular solitary freshwater hydrozoan polyp

(2) 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) A long faint constellation in the southern hemisphere near the equator stretching between Virgo and Cancer

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

"Hydra was slain by Hercules"