Third-party source

A third-party source (as opposed to first-party source or second-party source) is independent of the first party and second party in a transaction, interview, or written description (etc.). A first-party source comes directly from the first party (such as a self-published document or product description). A second-party source is controlled by the second party, such as a customer who has written some product requirements, or a prior book or article written by the same author.

In commerce, a "third-party source" means a supplier (or service provider) who is not directly controlled by either the seller (first party) or the customer/buyer (second party) in a business transaction. The third party is considered independent from the other two, even if hired by them, because not all control is vested in that connection. There can be multiple third-party sources with respect to a given transaction, between the first and second parties. A second-party source would be under direct control of the second party in the transaction.

In journalism, a "third-party source" is a document or entity which is not directly controlled by either the subject (first party) or the author (second party) of a written text. For example, a third-party source might be a prior news report about the primary subject (first party) of the writing. A second-party source could be a previous article already written by the same author (the second party) or by a group under direct control of that author. A first-party source might be an autobiography or product description written by the subject.

In Information Technology, a "third-party source" is a supplier of software (or a computer accessory) which is independent of the supplier and customer of the major computer product(s).

There are other uses of the term "third-party source" as well.

...
Wikipedia