problem vs hydra

problem and hydra both are nouns.

Word	Noun	Adjective	Verb	Adverb
problem	Yes	No	No	No
hydra	Yes	No	No	No

As nouns, hydra is a hyponym of problem; that is, hydra is a word with a more specific, narrower meaning than problem:

problem: a source of difficulty
hydra: trouble that cannot be overcome by a single effort because of its many aspects or its persistent and pervasive quality

Other hyponyms of problem include pressure point, can of worms, deep water, growing pains, matter.

problem (noun)	hydra (noun)
a source of difficulty	small tubular solitary freshwater hydrozoan polyp
a question raised for consideration or solution	trouble that cannot be overcome by a single effort because of its many aspects or its persistent and pervasive quality
a state of difficulty that needs to be resolved