A fictional universe is a self-consistent fictional setting which contains its own background elements including history and geography, and possibly also fantasy or science fiction elements such as magic or a means of faster than light travel, or other derived background elements. A fictional universe may also be called a fictional realm, imaginary realm, fictional world, imaginary world or imaginary universe. Most fictional universes are based directly or indirectly on our own universe, like the familiar "with a twist" world depicted in the popular Harry Potter series.

It is difficult to determine what actually constitutes a "fictional universe", but whether the universe is contained in a single work or consists of a succession of works, such as frequently appears in fantasy or science fiction series, the universe will be self-consistent and follow an established set of rules. Its history and geography are well-defined, and even languages may be constructed. When subsequent works are written within the same universe, care must be taken to ensure that established rules (canon) are not violated.

A virtual world is a computer-based simulated environment intended for its users to inhabit and interact via avatars. These avatars are usually depicted as textual, two-dimensional, or three-dimensional graphical representations, although other forms are possible (auditory and touch sensations for example). Some, but not all, virtual worlds allow for multiple users.

The computer accesses a computer-simulated world and presents perceptual stimuli to the user, who in turn can manipulate elements of the modeled world and thus experiences telepresence to a certain degree. Such modeled worlds may appear similar to the real world or instead depict fantasy worlds. The model world may simulate rules based on the real world or some hybrid fantasy world. Example rules are gravity, topography, locomotion, real-time actions, and communication. Communication between users has ranged from text, graphical icons, visual gesture, sound, and rarely, forms using touch and balance senses.

Massively multiplayer online games commonly depict a world similar to the real world, with real world rules and real-time actions, and communication. Communication is usually textual, with real-time voice communication using VOIP also possible.

Virtual worlds are not limited to games but, depending on the degree of immediacy presented, can encompass computer conferencing and text based chatrooms.