serving girl vs retainer

serving girl and retainer both are nouns.

Word	Noun	Adjective	Verb	Adverb
serving girl	Yes	No	No	No
retainer	Yes	No	No	No

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

serving girl: a girl who is a servant
retainer: a person working in the service of another (especially in the household)

Other hypernyms of serving girl include servant.

serving girl (noun)	retainer (noun)
a girl who is a servant	a dental appliance that holds teeth (or a prosthesis) in position after orthodontic treatment
	a person working in the service of another (especially in the household)
	a fee charged in advance to retain the services of someone