third party

noun

1. Someone not directly involved in a transaction; an entity beyond the seller (first party) and customer (second party).“A seller may employ a third party to perform specific services to augment the value of a product, such as packing and distribution.”
2. Someone only incidentally or tangentially connected to an incident or dispute; someone other than the principals; a bystander or independent witness.
3. A political party in opposition to the main parties in a two-party system