Elseworlds

Elseworlds was the publication imprint for American comic books produced by DC Comics for stories which took place outside the DC Universe canon. The Gotham by Gaslight graphic novel, featuring Batman, is considered to be the first official Elseworlds story. The "Elseworlds" name was trademarked in 1989, the same year as the first Elseworlds publication.

From 1942 to the mid-1980s, particularly during the 1960s — the Silver Age of Comic Books era — DC Comics began to make a distinction between the continuity of its fictional universe and stories with plots that did not fit that continuity. These out-of-continuity stories eventually came to be called Imaginary Stories.

The title page of "Superman, Cartoon Hero!" stated that the story was "Our first imaginary story", and continued to say: "In 1942, a series of Superman shorts started showing throughout the U.S.! So, with tongue firmly in cheek, the DC team turned out this story of what might have happened if Lois Lane had decided to see... Superman, Cartoon Hero!". The story opens with Lois determined to learn Superman's secret identity and going to the theater to see the Max Fleisher Superman short "Mad Scientist" in hopes of seeing the animated Man of Steel reveal his secret identity. In addition to other things, when the opening credits roll and state that the cartoons are based on DC Comics, Lois Lane states that she has never heard of DC Comics. Clark Kent then wonders if the people there are clairvoyant. In the final panel, Clark Kent exchanges a knowing wink with the image of himself as Superman on the movie screen.

Craig Shutt, author of the Comics Buyer's Guide column Ask Mr. Silver Age, states that true imaginary stories differed from stories that were dreams and hoaxes. Dreams and hoaxes were "gyps" on account of "not having happened" whilst true imaginary stories were canonical at least unto themselves. Also, since they were "just" imaginary and thus had no bearing on the characters’ regular stories, imaginary stories could show things like people dying and the victory of evil. In the optimistic and hopeful Silver Age of Comics, such stories usually wouldn't be told; this was hinted with writers telling readers how such an Imaginary Story often reassured the readers that it didn't really happen.

...
Wikipedia