An X-file is a case that has been deemed unsolvable by the Federal Bureau of Investigation. (TXF: "Daemonicus") The files constitute an unassigned project outside the Bureau mainstream that are more or less concerned with unexplained phenomena. (TXF: "Pilot") The X-Files Unit is the FBI department responsible for investigation of the X-files. (TXF: "Scary Monsters", "Jump the Shark")