noncitizen vs au pair

noncitizen and au pair both are nouns.

Word	Noun	Adjective	Verb	Adverb
noncitizen	Yes	No	No	No
au pair	Yes	No	No	No

As nouns, au pair is a hyponym of noncitizen; that is, au pair is a word with a more specific, narrower meaning than noncitizen:

noncitizen: a person who comes from a foreign country; someone who does not owe allegiance to your country
au pair: a young foreigner who lives with a family in return for doing light housework

Other hyponyms of noncitizen include deportee, exile, gringo, import, importee, metic.

noncitizen (noun)	au pair (noun)
a person who comes from a foreign country; someone who does not owe allegiance to your country	a young foreigner who lives with a family in return for doing light housework