third party

Short definition from Wiktionary

Noun

1. 1
   Someone not directly involved in a transaction; an entity beyond the seller (first party) and customer (second party).
2. 2
   Someone only incidentally or tangentially connected to an incident or dispute; someone other than the principals; a bystander or independent witness.
3. 3
   A political party in opposition to the main parties in a two-party system.

Adjective

1. 1
   Of or relating to a third party.
2. 2
   Of or relating to a third party: Of products, created or published by an independent publisher or manufacturer or its internal development teams, as opposed to first-party and second-party.

Verb

1. 1
   To intervene in a fight between two players or teams, often allowing one to easily defeat both (sometimes seen as unfair).