The Star Trek canon is the set of all canonical material in the Star Trek universe. The official Star Trek website defines canon as comprising the television series and feature films of the franchise.
As a rule, all Star Trek television series that aired are considered part of the canon.
This policy does not make clear which version of the series is the canonical one. For example, the remastered episodes of the original series, released in 2006, present several visual differences from the episodes originally aired.
Gene Roddenberry was something of a revisionist when it came to canonicity. People who worked with Roddenberry remember that he used to handle canonicity not on a series-by-series basis nor an episode-by-episode basis, but point by point. If he changed his mind on something, or if a fact in one episode contradicted what he considered to be a more important fact in another episode, he had no problem declaring that specific point not canonical.
See, people can easily catch us, and say "well, wait a minute, in 'Balance of Terror', they knew that the Romulans had a cloaking device, and then in 'The Enterprise Incident', they don't know anything about cloaking devices, but they're gonna steal this one because it's obviously just been developed, so how the hell do you explain that?" We can't. There are some things we just can't explain, especially when it comes from the third season. So, yes, third season is canon [sic] up to the point of contradiction, or where it's just so bad... you know, we kind of cringe when people ask us, "well, what happened in 'Plato's Stepchildren', and 'And the Children Shall Lead', and 'Spock's Brain', and so on—it's like, please, he wasn't even producing it at that point. But, generally, [the canon is] the original series, not really the animated, the first movie to a certain extent, the rest of the films in certain aspects but not in all... I know that it's very difficult to understand. It literally is point by point. I sometimes do not know how he's going to answer a question when I go into his office, I really do not always know, and—and I know it better probably than anybody, what it is that Gene likes and doesn't like.
— Richard Arnold, 1991
Another thing that makes canon a little confusing. Gene R. himself had a habit of decanonizing things. He didn't like the way the animated series turned out, so he proclaimed that it was not canon. He also didn't like a lot of the movies. So he didn't much consider them canon either. And – okay, I'm really going to scare you with this one – after he got TNG [Star Trek: The Next Generation] going, he... well... he sort of decided that some of The Original Series wasn't canon either. I had a discussion with him once, where I cited a couple things that were very clearly canon in The Original Series, and he told me he didn't think that way anymore, and that he now thought of TNG as canon wherever there was conflict between the two. He admitted it was revisionist thinking, but so be it.
— Paula Block, 2005
Additionally, David Gerrold, in an interview about Star Trek: The Animated Series, commented on Roddenberry's parsimony and how it originally affected the Star Trek's canon:
Arguments about "canon" are silly. I always felt that Star Trek Animated was part of Star Trek because Gene Roddenberry accepted the paycheck for it and put his name on the credits. And D. C. Fontana—and all the other writers involved—busted their butts to make it the best Star Trek they could. But this whole business of "canon" really originated with Gene's errand boy. Gene liked giving people titles instead of raises, so the errand boy got named "archivist" and apparently it went to his head. Gene handed him the responsibility of answering all fan questions, silly or otherwise, and he apparently let that go to his head.
Occasionally, writers will draw from non-canonical works in creating new canon. Such is the case of the first names for Hikaru Sulu and Nyota Uhura, which were first used in the novel The Entropy Effect and the reference book Star Trek II Biographies, respectively. Several concepts that first appeared in The Animated Series, which was considered to be non-canonical for several decades (1980s-2000s), were used in other Star Trek productions during that time, such as Kirk's middle name, first used in the episode "Bem" before it was used in Star Trek VI: The Undiscovered Country. The animated episode "Yesteryear" first introduced The Forge and the city of ShiKahr, which were later included in the Enterprise three-part story that started with "The Forge".
One final issue comes from text that appears on props such as computer displays, but is not legible during the episode, except in modern HDTV broadcasts. The transcript of the text can often be obtained through behind-the-scenes pictures and interviews. This leads to the question of whether material that is in the episodes but cannot be seen clearly should be considered canon. Often, this material tends to be inside jokes inserted by the production staff. Other kinds of information, such as the biographical information seen on a computer display in the Enterprise episode "In a Mirror, Darkly", has been stated to not be "hard canon".
All official Star Trek feature films are also considered canonical. While not explicitly stated, the most complete released version of the films, including scenes missing from the theatrical version of a film but included in home releases or director's cuts, appear to be canonical. One scene, deleted from Star Trek II: The Wrath of Khan, revealed Peter Preston as the nephew of Scotty. Peter Preston is included in the canon database at StarTrek.com.
Adding confusion to the issue is the fact that Roddenberry is quoted as saying he disliked the films, and "didn't much consider them canon". There exists no definitive list of which films in particular Roddenberry disliked, or what elements in them he did not consider canonical. For example, the reference book Star Trek Chronology states that Roddenberry considered elements of Star Trek V and Star Trek VI to be apocryphal, but it does not specify which particular elements in the films Roddenberry objected to.
The canonicity of extra features found on home DVD releases, such as deleted scenes, has never been explicitly addressed.
Star Trek (the 2009 film), Into Darkness and Beyond occur in a separate timeline from the rest of the series. In June 2016, for the computer game Star Trek Online, CBS named this the Kelvin Timeline, after the USS Kelvin which was attacked and destroyed in the opening scene of Star Trek. Former names for this universe have included the alternate timeline and the reboot series.
Events depicted in the Kelvin Timeline films that occur prior to the temporal incursion that creates the new timeline, such as the destruction of the planet Romulus, have been referenced in later "prime timeline" works such as Star Trek: Picard and Star Trek: Discovery.
Many of the original novels published by Pocket Books are not considered part of the canon. This was a guideline set early on by Gene Roddenberry, and repeated many times by people who worked with him.
And as long as Gene Roddenberry is involved in it, he is the final word on what is Star Trek. So, for us here – Ron Moore, Jeri Taylor, everybody who works on the show – Gene is the authority. And when he says that the books, and the games, and the comics and everything else, are not gospel, but are only additional Star Trek based on his Star Trek but not part of the actual Star Trek universe that he created... they're just, you know, kinda fun to keep you occupied between episodes and between movies, whatever... but he does not want that to be considered to be sources of information for writers, working on this show, he doesn't want it to be considered part of the canon by anybody working on any other projects.
— Richard Arnold, 1991
However, even this rule is not without rare exceptions. Two Voyager novels written by Jeri Taylor (co-creator and then producer of Voyager), Mosaic and Pathways, were written early on in Voyager's run and detailed the background of the series' main characters. These were meant to be canon, and to be used as references by the series' writers when fleshing out the characters. These two novels are sometimes named as exceptions to the "no book is canon" rule. However, as some of the background information mentioned in those books was never referenced in an episode of Voyager, or was contradicted in episodes written after they were published, their status as canon is still open to debate.
The novelizations of episodes and movies are not considered canon. This is a tradition that also goes back to Roddenberry himself. His novelization of Star Trek: The Motion Picture includes many tangents and new information. It reveals, for instance, that the woman who dies in the transporter accident was Kirk's former spouse. While this novel filled in many gaps left in the movie, Roddenberry is quoted as saying it should not be considered canon.
There are conflicting messages concerning "non-fiction" reference books such as The Star Trek Encyclopedia, Star Trek Chronology, Star Trek: The Next Generation Technical Manual and Star Trek: Deep Space Nine Technical Manual. Unlike the novels and novelizations, these reference manuals have never been explicitly named as non-canon, and the fact that they were officially sanctioned by Paramount and given to episode writers as guides serves to give them an aura of credibility. Roddenberry himself considered it part of the "background" of Star Trek. Meanwhile, Michael Okuda and Rick Sternbach, artists and technical consultants since Star Trek: The Next Generation and the authors of several of these reference books, considered their work "pretty official". However, they stop short of naming the books canon, leaving the debate open.
Star Trek writer and co-producer Ronald D. Moore dismisses such official material as "speculation", and says that the writing staff did not consider it canon. However, In a series of posts to the official Star Trek website's forums, Viacom Senior Director Harry Lang left no doubt that he considers the reference books as canon.
Star Trek comic books and magazines are generally not considered part of the canon. Regarding IDW Publishing's comic book tie-ins to the 2009 film and its sequel, screenwriter Roberto Orci felt that the background information conveyed in those books could be considered canonically accurate. Using rules similar to the ones that governed Star Wars canon at the time, he acknowledged that the extended universe material he oversees could remain part of the accepted canon unless contradicted by future films or television series.
Nothing that takes place in Star Trek games, the Star Trek: The Experience attraction, Star Trek fan productions or Trekdom is considered part of the canon.
Based on the amount of creative control Roddenberry exerted over the first seasons of Star Trek, some people argue that only Roddenberry-approved material should be considered canonical. Such an approach would eliminate from the canon anything Roddenberry disliked, as well as everything made after his death, including seven movies and multiple television series.
However, Roddenberry himself preemptively rebuked such an attitude. He had hoped that Star Trek would go on after his death. As Star Trek was constantly improved by each following generation, he expected people to look back upon its humble beginnings as just that, the simple beginnings of something much bigger and better.
The Klingon language was first conceived by James Doohan for the movie Star Trek: The Motion Picture, and consisted only of a few words. Later, Marc Okrand proceeded to flesh out the sparse vocabulary into a real language, complete with grammar rules and phonology, and went so far as to publish The Klingon Dictionary (1985, revised edition 1992); the Klingon Language Institute was created soon thereafter. Okrand's Klingon language was used to write the Klingon dialogues heard in several Star Trek movies and episodes. Okrand has developed the language in an important way in two audio-courses: Conversational Klingon (1992) and Power Klingon (1993), and in two books: The Klingon Way (1996) and Klingon for the Galactic Traveller (1997). Despite these facts, however, Ronald D. Moore stated in 1997: "Whether or not [Trek writers] use the language as spelled out in Marc's dictionary is up to the individual writer," and that he "find[s] the dictionary cumbersome and usually find[s] it easier to make [the language] up phonetically."