32 CFR 257.3: Definition.

Service of Process. When applied to the filing of a court action against an officer or agency of the United States, service of process refers to the delivery or, when appropriate, receipt by mail, of a summons and complaint made in accordance with Rule 4, Federal Rules of Civil Procedure by serving the United States and by serving a copy of the summons and complaint by registered or certified mail to such officer or agency. It further signifies the delivery of a subpoena requiring a witness to appear and give testimony or of a subpoena requiring production of documents, or delivery of a subpoena for any other reason whether or not the matter involves the United States.