"In biblical scholarship, Q is a hypothetical source that both Matthew and Luke supposedly used."
https://www.catholic.com/magazine/online-edition/some-thoughts-on-q
"The Gospel of Q remains a hypothetical document. No intact copy has ever been found. No reference to the document in early Christian writings has survived. Its existence is inferred from an analysis of the text of Matthew and Luke.
Much of the content of Matthew and Luke were derived from the Gospel of Mark. But there were also many passages which appear to have come from Q."
http://www.religioustolerance.org/gosp_q.htm
"According to the Two Source Hypothesis accepted by a majority of contemporary scholars, the authors of Matthew and Luke each made use of two different sources: the Gospel of Mark and a non-extant second source termed Q."
http://www.earlychristianwritings.com/q.html
"Q is the designation for a gospel that no longer exists, but many think must have existed at one time. In fact, even though no copy of this gospel has survived independently, some nineteenth-century scholars found fragments of such an early Christian composition embeded in the gospels of Matthew and Luke."
https://www.pbs.org/wgbh/pages/frontline/shows/religion/story/qthomas.html
"The Q source (also called Q document, Q Gospel, or Q from German: Quelle, meaning "source") is a hypothetical written collection of primarily Jesus' sayings (logia). Q is part of the common material found in the Gospels of Matthew and Luke but not in the Gospel of Mark. According to this hypothesis, this material was drawn from the early Church's oral tradition."
https://en.wikipedia.org/wiki/Q_source
"The working hypothesis is that Matthew and Luke, in addition to having Mark as a source for their information, had a second independent source that Mark did not use. This second independent source is called simply the "Q-source."
That letter Q is used since it is the first letter of the German word quelle, which is simply the word for source. That is to say, the Q-source is a source that is unknown to us but known to the gospel writers Matthew and Luke."
https://www.christianity.com/jesus/is-jesus-god/the-gospels/what-is-the-q-source.html
"The whole idea of a Q gospel is based on the concept that the Synoptic Gospels (Matthew, Mark, and Luke) are so similar that they must have copied from each other and/or another source. This other source has been given the name "Q.""
https://www.gotquestions.org/Q-Gospel.html
"Q is the designation given to a hypothetical sayings source that many scholars believe was incorporated into the Gospels of Luke and Matthew. Though some notable scholars have questioned the theory, others have proposed reconstructions of Q based on a careful comparison of New Testament Gospels.
Since no manuscripts of Q have survived from antiquity, the translation is based on the Greek text printed in The Critical Edition of Q."
https://www.gospels.net/quelle
What is the Q 'Gospel'? The Gospel According to 'St Q'?
https://bible.org/seriespage/7-what-q-gospel-gospel-according-st-q