The canon of a work of fiction is "the body of works taking place in a particular fictional world that are widely considered to be official or authoritative; [especially] those created by the original author or developer of the world". Canon is contrasted with, or used as the basis for, works of fan fiction and other derivative works.

When there are multiple "official" works or original media, what material is canonical can be unclear. This is resolved either by explicitly excluding certain media from the status of canon (as in the case of Star Trek and Star Wars); by assigning different levels of canonicity to different media; by considering different but licensed media treatments official and equally canonical to the series timeline within their own continuities' universe, but not across them; or not resolved at all.

There is also no consensus regarding who has the authority to decide what is or isn't canonical, with copyright holders usually declaring themselves the authorities when they want to erase or retcon materials that were approved by the setting's original creator (with Star Wars again being an example). The definition of canon is of particular importance with regard to reboots or re-imaginings of established franchises, such as the Star Trek remake (2009), because of the ways in which it influences the viewer experience.

The official Star Trek website describes the Star Trek canon as "the events that take place within the episodes and movies", referring to the live-action television series and films, with Star Trek: The Animated Series having long existed in a nebulous gray area of canonicity. Events, characters, and storylines from tie-in novels, comic books, and video games are explicitly excluded from the Star Trek canon, but the site notes that elements from these sources have been subsequently introduced into the television series, and says that "canon is not something set in stone".

During George Lucas's time with the franchise, the Star Wars canon was divided into discrete tiers that incorporated the Expanded Universe (EU), with continuity tracked by Lucasfilm creative executive Leland Chee. Higher-tier and newer material abrogated lower-tier and older material in case of contradiction. The live-action theatrical films, the 2008 The Clone Wars TV series and its debut film, and statements by Lucas himself were at the top of this hierarchy; such works invariably superseded EU material in case of contradiction. The EU itself was further divided into several descending levels of continuity. After Disney's acquisition of the franchise, Lucasfilm designated all Expanded Universe material published before 25 April 2014 (other than the first six theatrical films and the 2008 The Clone Wars film and TV series) as the non-canonical "Legends" continuity. Material released since this announcement is a separate canonical timeline from the original George Lucas Canon, with all narrative development overseen by the Lucasfilm Story Group.

The makers of Doctor Who have generally avoided making pronouncements about canonicity, with Russell T Davies explaining that he does not think about the concept for the Doctor Who television series or its spin-offs.

The television series The Simpsons has as an example of non-canonical material the Treehouse of Horror episodes, a series of Halloween-themed specials with several stories that take place outside the show's normal continuity.

Productions under Walt Disney Animation Studios are considered by The Walt Disney Company as the canonical stories under the company's "Disney" brand. Therefore, sequels, prequels, television series, or other related media produced by other studios of the company (as is the case of Disneytoon Studios and Disney Television Animation productions that serve as a continuity of the films), and events in other media like books or video games, are ignored within the main Disney brand, focused on productions under Walt Disney Animation Studios, thus other productions and media being considered non-canonical events.^{[better source needed]}