female person vs girl

female person and girl both are nouns.

Word	Noun	Adjective	Verb	Adverb
female person	Yes	No	No	No
girl	Yes	No	No	No

As nouns, girl is a hyponym of female person; that is, girl is a word with a more specific, narrower meaning than female person:

female person (noun)	girl (noun)
a person who belongs to the sex that can have babies	a young female
	a girl or young woman with whom a man is romantically involved
	a youthful female person
	a friendly informal reference to a grown woman
	a female human offspring