It is somone other than the first two principals who have entered into a contract or an agreement. Third party are not a direct party in the agreement or contract, but they may be present as a beneficiary to the contract or somone affected by the contract between the first two principals (first party and second party).
Or, software extension. A software extension is a computer program designed to be incorporated into another piece of software in order to enhance, or extend, the functionalities of the latter. On its own, the program is not useful or functional.