Canon is the body of admissible evidence for any particular universe. The age-old question of what is and what is not canon is one of the most salient points of the debate, and has perhaps surpassed the actual technical debates as the key issue in the whole Star Wars vs. Star Trek argument.
What is canon?
Historically, fans have had unusual difficulty in understanding precisely what is and is not canon. This dispute has likely occurred because fans and writers have often defined canon differently.
Canon refers to all admissible evidence. If two canon "facts" disagree with each other and no rationalization can be found, then only one of the two canon facts would be considered a part of continuity.
Writers are often more concerned about what is a part of continuity than what is canon, and tend to answer questions with that in mind. For example, in the canon novelization of A New Hope, Luke was designated Blue 5. In the canon movie, he was Red 5. The "Red 5" designation is considered a part of continuity while "Blue 5" is not, even though both conflicting facts are canon. (See the Star Wars canon section below for more details on how Star Wars canon is determined.)
Star Wars canon
There are four main levels of canon within the Star Wars universe, which work together to create what is known as the overall continuity of the Star Wars universe. Material outside the films is referred to as the Expanded Universe, or EU for short.
- This consists of the six Star Wars films, and adaptations of them such as novelizations and comics. Nothing is permitted to contradict these in any way, at least not while remaining in the overall continuity.
- The vast majority of Star Wars material, such as novels, short story collections, comics and selected video games. C-Canon material cannot contradict G-Canon material, but it is quite possible for the reverse to happen. In practice though, this is not likely to happen in the future, unless the proposed Star Wars TV series is designated G-Canon. The Incredible Cross-Sections book series (which helped spark the current iteration of the continuity debate) falls into this category.
- Usually made up of older or less serious material. S-Canon material still counts in the overall continuity, so long as it is not contradicted by anything at the G or C levels.
- Material in this category is not part of the Star Wars continuity, either because it has been contradicted by the films, or because it was meant to take place in an alternate timeline, for instance. There's some debate about whether the original, pre-Special Edition films belong here; some say that Lucasfilm has decanonized the originals by withdrawing them from distribution (though they've since made them available on DVD again). On the other hand, there are also those who argue that both are equally valid versions, or even that the Special Editions are in fact the N-Canon versions, as they are obviously edited versions of the originals.
A rigorous checking process is employed to ensure that all new C-Canon material is in compliance with other G and C-Canon material (keeping continuity with S-Canon is less important). In some cases, works have been delayed while continuity problems are sorted out. This, and the fact that there is a specific category for non-canon material, shows that Lucasfilm and its subsidiaries consider most of their material to be part of an interlocking canon, which they try (if not always succeeding) to preserve.
Some forms of media are given special treatment in order to fit with continuity. For example, the gameplay mechanics of video games are not considered a part of continuity, while the overall storyline usually is.
Star Trek canon
The creator of Star Trek, Gene Roddenberry once said that "It isn't Star Trek until I say it's Star Trek." This statement generally isn't taken seriously, however, as it would render all material made since his death in late 1991 non-canon. In practice, only on-screen, live-action material from the ten films and five television series is considered to be canon.
Star Trek also has its own novel series. The rules for this are defined in the submission guide for Pocket Books (the arm of Paramount in charge of the novels), which states that continuity with the films and shows must always be respected, and while contradicting other novels is discouraged, it's not actually forbidden.
Some confusion was raised when Star Trek: Voyager producer Jeri Taylor wrote two novels (Mosaic and Pathways) and declared them to be canon. This was mainly for the purposes of providing background for Captain Janeway; the details in the book were not picked up on by other writers, however, and parts of the book were later contradicted outright on the show, firmly establishing the book as non-canon, an establishment strengthened when a statement was released reinforcing the fact that all the Star Trek books are effectively non-canon.
The Canon Debate
Despite both Lucasfilm and Paramount making relatively clear statements on the canon and continuity of their respective series', some have chosen to argue against these positions. The most common claim is that no Star Wars material other than the films is canon, and this is usually "supported" by a quote by George Lucas, saying that EU material is supposedly in a "parallel universe" and that the novels "don't intrude on the films, but intrude between them." While this is generally understood as meaning that the novels and other official materials are not allowed to contradict the films but are otherwise entirely valid, some have chosen to interpret Lucas' statements as meaning that the novels take place in a separate reality, and therefore (for some reason) have no validity.
There are also those who try and claim that the Star Trek novels are canon, but this position is much rarer, as the Trek novels being canon would not greatly benefit the overall chances of the Star Trek side.