third party

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A political party organized as opposition to the existing parties in a two-party system.
• noun One other than the principals involved in a transaction.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun someone not directly involved in a transaction. A third entity in the Seller (first party) and Customer (second party) relationship. A Seller may employ a third party to perform specific services to augment the value of a product. For example, a manufacturer may employ a third party to pack and distribute a product. A computer manufacturer may augment their product with software from a third-party supplier.
• noun someone only incidentally or tangentially connected to an incident or dispute; someone other than the principals; a bystander or independent witness.
• noun a political party in opposition to the main parties in a two-party system

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone other than the principals who are involved in a transaction
• noun a political party organized in opposition to the major parties in a two-party system