arbiter vs third party

arbiter and third party both are nouns.

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

As nouns, third party is a hyponym of arbiter; that is, third party is a word with a more specific, narrower meaning than arbiter:

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

arbiter (noun)	third party (noun)
someone chosen to judge and decide a disputed issue	a political party organized in opposition to the major parties in a two-party system
someone with the power to settle matters at will	someone other than the principals who are involved in a transaction