Third-Party Summons is a legal term that refers to the process of having a third party, such as a court server, deliver a legal document to the defendant in a lawsuit. The document usually contains details of the lawsuit and a court order to appear in court on a certain date and time. There are two types of Third-Party Summons: service of process and substituted service of process. Service of process involves a third party delivering the summons in person to the defendant. Substituted service of process is when a third party delivers the summons to someone else, such as a family member, in order to notify the defendant of the lawsuit.