Definition from Wiktionary, the free dictionary
- 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.
- someone only incidentally or tangentially connected to an incident or dispute; someone other than the principals; a bystander or independent witness.
- a political party in opposition to the main parties in a two-party system
someone not directly involved in a transaction.
a political party in opposition to the main parties in a two-party system
Derived terms 
- third-party (adjective)