It is impossible to precisely define what science fiction is. The generally accepted definition states that science fiction is any type of fiction where the setting is a world that is different from the world the writer lives in, and that this difference is a result of a reasonable and explainable process (usually advances in technology). In order for the work to be considered science fiction, this setting must form an integral part of the storyline. Many authors agree that if the plot of a science fiction story can be successfully transplanted into another setting, the story cannot truly be considered to be science fiction. The difference in the setting of the story must have a large impact on the characters of the piece.

Science fiction is often compared to fantasy writing, as both forms revolve around a setting that differs greatly from the world of the writer. It is true that fantasy and science fiction writing share several attributes, the main difference between the two is that science fiction occurs in a setting that could be reasonably foreseen, while fantasy writing involves a world that could not reasonably occur.