California Code, Code of Civil Procedure - CCP § 428.70

(1) “Third-party plaintiff” means a person against whom a cause of action has been asserted in a complaint or cross-complaint, who claims the right to recover all or part of any amounts for which he may be held liable on such cause of action from a third person, and who files a cross-complaint stating such claim as a cause of action against the third person.

(2) “Third-party defendant” means the person who is alleged in a cross-complaint filed by a third-party plaintiff to be liable to the third-party plaintiff if the third-party plaintiff is held liable on the claim against him.