third party

English Thesaurus

1. a political party organized in opposition to the major parties in a two-party system (noun.group)

hypernym	:	party, political party,
definition	:	an organization to gain political power (noun.group)

2. someone other than the principals who are involved in a transaction (noun.person)

hypernym	:	arbiter, arbitrator, umpire,
definition	:	someone chosen to judge and decide a disputed issue (noun.person)