Legal Definition of Third party: (a) Someone who is not directly connected with a contract, a deal, a lawsuit, an occurrence, etcetera, but who may be affected by its outcome. (b) Persons other than the plaintiff or defendant who are brought into a case | Looking to understand the legal definition of a third party? Learn how it impacts your business, its importance, and examples that clarify its scope. Gain insights to protect your operations and maintain positive relationships