1.nounSomeone 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.nounSomeone only incidentally or tangentially connected to an incident or dispute; someone other than the principals; a bystander or independent witness.
3.nounA political party in opposition to the main parties in a two-party system