sylph vs adult female

sylph and adult female both are nouns.

Word	Noun	Adjective	Verb	Adverb
sylph	Yes	No	No	No
adult female	Yes	No	No	No

As nouns, adult female is a hypernym of sylph; that is, adult female is a word with a broader meaning than sylph:

Other hypernyms of sylph include woman.

sylph (noun)	adult female (noun)
an elemental being believed to inhabit the air	an adult female person (as opposed to a man)
a slender graceful young woman