The Star Trek Expanded Universe is an official, created term to describe an extrapolation of events which occur in the Star Trek Universe outside the scope of the television series, feature films and novels. Information from the Star Trek "Expanded Universe" typically fills "holes" and is considered to be “canon” in the Star Trek story and timeline, with explanations of events which have never been adequately explained through live action productions. The term was first used in 1966 by writer D.C. Fontana to describe information put forth in the backstory of Doctor Leonard McCoy.
Although original Star Trek fiction (for adult audiences) dates back to James Blish's 1970 novel Spock Must Die!, published by Bantam Books, the publishing company Simon & Schuster is most directly responsible for contributing to the Star Trek Expanded Universe through its license with Pocket Books which has generated many Star Trek novels over the past twenty-five years. Information in the novels, while sometime contradictory, often serves to provide information to the Star Trek Expanded Universe.
The term "Star Trek Expanded Universe" is not an official usage of Paramount Pictures, Simon & Schuster, or any other Star Trek licensee. The term is occasionally used within fandom by analogy with the Star Wars expanded universe as defined by Lucasfilm. However, the policies and practices pertaining to licensed Star Trek fiction are not identical to those used by Lucasfilm. For example, unlike in Star Wars fiction, there is no overall effort among the various licensees to integrate all Star Trek fiction into a single continuity. Although the majority of Star Trek novels in recent years have been written to be consistent with one another and with certain Star Trek comic books, they remain incompatible with many earlier novels and comics, and occasionally with contemporary novels as well.
Many of these premises have been accepted by Star Trek fans as being canon, even though Paramount Pictures, owners of the Star Trek franchise, considers only live-action television and film productions to be canon. This has led to conflict on occasion when a TV episode or film contradicts well-established Expanded Universe backstory (most notably in the case of the prequel series, Star Trek: Enterprise).
Star Trek: The Animated Series (TAS) was a 22 episode set of animated adventures of the Enterprise crew, which aired originally on NBC from 1973-1974. It featured the reappearance of popular characters from the original series, including Harry Mudd and Cyrano Jones, and speculates on what happened to them after they were last seen in live-action. Some Star Trek novels and comics have utilized characters that were only ever seen in the animated series, most notably Arex and M'Ress. It has been suggested by fans that these stories took place during the fourth or fifth year of Kirk's original "five-year mission." Arguably, the most-debated element of TAS is the introduction of Robert April as the first captain of the Enterprise NCC-1701.
From the show's debut up until the end of the first season of Star Trek: The Next Generation, TAS was considered part of Star Trek canon. Then, from the end of the first season of Star Trek: The Next Generation on May 16, 1988 (when all licenses for Star Trek spin-off fiction were renegotiated) up to June 27, 2007, TAS was "decanonized". However, on June 27, 2007, Star Trek's official website incorporated information from TAS into its library, recanonizing it.