Fountain of Youth vs fountain

Fountain of Youth and fountain both are nouns.

Word	Noun	Adjective	Verb	Adverb
Fountain of Youth	Yes	No	No	No
fountain	Yes	No	No	No

As nouns, fountain is a hypernym of Fountain of Youth; that is, fountain is a word with a broader meaning than Fountain of Youth:

Fountain of Youth: a fountain described in folk tales as able to make people young again
fountain: a natural flow of ground water

Other hypernyms of Fountain of Youth include natural spring, outflow, outpouring, spring.

Fountain of Youth (noun)	fountain (noun)
a fountain described in folk tales as able to make people young again	a plumbing fixture that provides a flow of water
	an artificially produced flow of water
	a structure from which an artificially produced jet of water arises
	a natural flow of ground water