Defining mainstream fiction is not very difficult. It is fiction that might take place in the real world, either today or in the past. It involves no magic (although psychic phenomena might appear, since some people believe they are real). Similarly, while technology in mainstream fiction must not violate known principles of science, it can be something that does not exist, such as commonplace flying cars.

Historical fiction need not conform strictly to the actual historical record. A prime example I am familiar with is The Man in the High Castle by Philip K. Dick, in which the Nazis won World War II and conquered America. Such "alternate-history" tales explore what might result if some important event, such as Abraham Lincoln's assassination, had not happened or had turned out differently.

The great majority of mainstream fiction, of course, simply has ordinary characters interacting in ordinary ways — for example cheating on their spouses, or committing murder and trying to avoid punishment. I am less likely to read such works, although I have enjoyed some (as this list indicates.)