third party vs arbitrator

third party and arbitrator both are nouns.

Word	Noun	Adjective	Verb	Adverb
third party	Yes	No	No	No
arbitrator	Yes	No	No	No

As nouns, arbitrator is a hypernym of third party; that is, arbitrator is a word with a broader meaning than third party:

third party: someone other than the principals who are involved in a transaction
arbitrator: someone chosen to judge and decide a disputed issue

Other hypernyms of third party include arbiter, umpire.

third party (noun)	arbitrator (noun)
a political party organized in opposition to the major parties in a two-party system	someone chosen to judge and decide a disputed issue
someone other than the principals who are involved in a transaction