In the context of fiction, apocrypha includes those fictional stories that do not belong within a fictional universe's canon, yet still have some authority relating to that fictional universe. The boundaries between canon and apocrypha can often be blurred.
The word "Apocrypha" is sometimes used to describe works set in a fictional universe that may not belong in the canon.
These may include tie-in merchandise such as video games, novels and comics, which are sometimes termed 'Expanded Universes'.
Often these materials might contradict the continuity that has already been established by 'canon'. Even when no such contradictions occur, such materials may still be deemed apocrypha possibly because they might have been produced largely independently of the creator of the fictional universe. For example, Joss Whedon, creator of the Buffyverse, has little involvement with the Buffyverse novels and has never read an entire novel, let alone closely overseen or edited one.
The Star Trek canon consists of the various Star Trek television series and movies. All the other Star Trek stories which have been licensed by Paramount (novels, comics..) are not part of canon, they are instead apocrypha. Fan fiction is classified as fanon.
Etymology
The word derives the Greek word 'απόκρυφα, which means "opened", "revealed", or "uncovered". Jerome coined the term to refer to those books of the Old Testament that were not found in the Hebrew Tanakh, and it has since been used to describe religious texts that may not belong to the canon.
Use of the term has extended to non-religious contexts, where an account or anecdote is said to be apocryphal if its authenticity is questionable (more often when the account's veracity is probably questionable). In recent years it has sometimes been used in the context of fiction.