Canonicity
When there are multiple "official" works or original media, the question of what is and what is not 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), by assigning different levels of canonicity to different media (as in the case of Star Wars), by considering different but licensed media treatments official within their own continuities but not across them (as with Battlestar Galactica) or not resolved at all.
The official Star Trek website describes Star Trek canon as "the events that take place within the live-action episodes and movies" (that is, the television series Star Trek, Star Trek: The Next Generation, Star Trek: Deep Space Nine, Star Trek: Voyager, Star Trek: Enterprise, and the Star Trek motion pictures). Events, characters and story lines from tie-in novels, comic books, video games and Star Trek: The Animated Series 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." One example of a non-canonical element that later became canonical in the Star Trek universe was the name "Tiberius" becoming the official middle name of Enterprise captain James T. Kirk. The name was introduced in the Star Trek animated series, and was later added into the official biography of the character by its mention in the live-action film Star Trek VI: The Undiscovered Country.
The Star Wars canon exists on several levels. The highest level is the six Star Wars films, and statements by George Lucas; tie-in fiction from the Expanded Universe has a different level of canonicity. The complex system is maintained by Leland Chee, a Lucasfilm employee.
Read more about this topic: Canon (fiction)