mill-girl vs girl

mill-girl and girl both are nouns.

Word	Noun	Adjective	Verb	Adverb
mill-girl	Yes	No	No	No
girl	Yes	No	No	No

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

Other hypernyms of mill-girl include fille, miss, missy, young lady, young woman, factory worker, mill-hand.

mill-girl (noun)	girl (noun)
a girl who works in a mill	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