The Q Gospel (or "Q Source") is a never-discovered document that some feel was the source for material included in both the Gospel of Matthew and the Gospel of Luke, but is absent from the Gospel of Mark.
Advocates for an existence of a Q Gospel tend to be deniers of the inspired nature of the Gospels, and tend to argue that the Gospels were written later (after A.D. 70) rather than earlier (before A.D. 65).[1]