A fictional universe is a self-consistent fictional setting with elements that differ from the real world. It may also be called an imagined, constructed or fictional realm (or world).

The terms multiverse, parallel universe, alternate history, story or screen bible, backstory and crossover have a considerable amount of overlap with fictional universes.

A fictional universe can be almost indistinguishable from the real world, except for the presence of the invented characters and events that characterize a work of fiction; at the other extreme it can bear little or no resemblance to reality, with invented fundamental principles of space and time.

The subject is most commonly addressed in reference to fictional universes that differ markedly from reality, such as those that introduce entire fictional cities, countries, or even planets, or those that contradict commonly known facts about the world and its history, or those that feature fantasy or science fiction concepts such as magic or faster than light travel—and especially those in which the deliberate development of the setting is a substantial focus of the work.