foreigner vs au pair

foreigner and au pair both are nouns.

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

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

foreigner: 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 foreigner include deportee, exile, gringo, import, importee, metic.

foreigner (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
someone who is excluded from or is not a member of a group