A hypothetical book, which is asserted by a scholar of the Jesus Seminar to be the "uncensored" version of the gospel of Mark. There is a rather poor case for this book, and an even worse case for its authenticity.

All we have is one scholar who found a quotation (now lost) from Clement of Alexandria which supposedly comes from this gospel. Further, we don't have any reason to think the quote is valid or authentic. Worse, Clement apparently has a history of accepting untrustworthy sources.

So, the Secret Gospel of Mark is something we don't have, quoted by a text we don't have, by an author whose works are questionable.

Also see: Jesus Seminar, What's wrong with the Jesus Seminar?