tomboy vs girl

tomboy and girl both are nouns.

Word	Noun	Adjective	Verb	Adverb
tomboy	Yes	No	No	No
girl	Yes	No	No	No

As nouns, girl is a hypernym of tomboy; that is, girl is a word with a broader meaning than tomboy:

Other hypernyms of tomboy include fille, miss, missy, young lady, young woman.

tomboy (noun)	girl (noun)
a girl who behaves in a boyish manner	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