The defendant in a civil lawsuit is formally notified through service of process, a legal procedure designed to ensure due process and fair notice under the U.S. Constitution. This process involves delivering two key documents: a summons (a court order requiring the defendant to respond) and a copy of the complaint (the document outlining the plaintiff’s allegations and claims). 

The rules for service are governed by Federal Rule of Civil Procedure (FRCP) 4 in federal courts and analogous state laws, which prioritize methods that ensure the defendant actually receives notice