Star Trek canon
The Star Trek canon is the set of all material taking place within the Star Trek universe that is considered official. The definition and scope of the Star Trek canon has changed over time. Until late 2006, it was mainly composed of the live-action television series and films before becoming a more vague and abstract concept. From 2010 until 2023, the official Star Trek website's site map described their database, which listed both animated and live-action series and films as its sources, as "The Official Star Trek 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. 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. Television series and films
All Star Trek live-action television series and films are and always have been generally considered part of the canon, while Star Trek: Lower Decks and Star Trek: Prodigy are both fully accepted as canonical as well. Until 2010, everything outside of the live-action television episodes and films were "traditionally" considered non-canonical, including The Animated Series. However, large portions of the fan base, as well as Star Trek affiliates, supported The Animated Series being adopted as fully canonical. With the relaunch of StarTrek.com in 2010, The Animated Series was added to the list of canonical shows included in the database, thus officially confirming the show's new status as part of the Star Trek canon. Gene Roddenberry was something of a revisionist when it came to the canon. People who worked with Roddenberry have remembered that he used to handle canonicity on a point-by-point basis rather than series-by-series or episode-by-episode. 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 fact not canonical. 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. Other licensed works
In general, Star Trek novels 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:

However, 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 meant to be canonical, to be used as reference materials for use by Voyagers writers. 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 within the canon is still open to debate. There are also 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-canonical, 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. Similarly, 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 canonical, leaving the debate open. Star Trek writer and co-producer Ronald D. Moore dismissed such material, saying that, although the writing staff would often consult reference materials, they did not consider them canonical, reserving that title for the episodes and films. However, in a series of posts to the official Star Trek website's forums, Viacom Senior Director Harry Lang established his opinion that the reference books are canonical, saying "Only the reference books (tech manual, encyclopedia, etc...) and two books by Jeri Taylor are considered canon outside the tv show and movies." The novelizations of episodes and movies are not considered canonical. 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, it has been said that Roddenberry himself thought it should not be considered canonical:

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 the 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, or any other licensed material is considered canonical, nor are any unlicensed works such as Star Trek fan productions. See also
Timeline of Star Trek
Outline of Star Trek

References

External links
What is considered Star Trek "canon"? at StarTrek.com

What is Canon? at Ex Astris Scientia

Canon
Canons (fiction)