first-party adjective 3first party noun 2forms 1 adjective3 •Of or relating to someone (a first party) directly involved in a given transaction, such as a buyer or seller. •Of products, created or published by the company responsible for the platform on which it is released (or its internal developers), as opposed to third-party. •Of or relating to the plaintiff in a lawsuit. first party noun2 •An individual or entity directly involved in a given transaction, such as a buyer or seller. •The plaintiff in a lawsuit. formsfirst parties