Of or relating to someone (a first party) directly involved in a given transaction, such as a buyer or seller. / Of or relating to the plaintiff in a lawsuit.

(business, commerce) An individual or entity directly involved in a given transaction, such as a buyer or seller. / (law) The plaintiff in a lawsuit.

plural of first party

The situation where a company producing goods also maintains and operates its own vehicles for delivery, rather than using third-party carriers.