What is Gaiden?
1.
A side story or a spinoff of of a published work. It is not considered as a prequel or a sequel. Some gaidens are just stories retold from another character's perspective, or a flashback.
In a gaiden of Dragon Knights, by Mineko Ohkami, Rune has a nightmare of everyone becoming crossdressers.
See
2.
Japanese. Means a compilation or collection.
Final Fantasy IX is sometimes called a gaiden by its fans because it borrows a lot from older Final Fantasies.
