The document that commands a Defendant to appear in Court and defend themselves in a lawsuit is known as the Complaint.

Actually, the document that commands a defendant to appear in court and respond to a lawsuit is typically called a "Summons," not a "Complaint."

In legal terms:

1. Complaint: This is the initial document filed by the plaintiff (the party bringing the lawsuit) that outlines the allegations, legal claims, and demands against the defendant. It provides the basis for the lawsuit and what the plaintiff is seeking.
2. Summons: The summons is a document issued by the court and served on the defendant, notifying them that a lawsuit has been filed against them and instructing them to appear in court to respond to the allegations in the complaint.

In summary, the complaint initiates the legal action by outlining the plaintiff's claims, while the summons is the official notice to the defendant, instructing them to respond to those claims in court.