Gaiden

Gaiden (外伝?, ) is a Japanese-language word meaning "side story" or "tale", used to refer to an anecdote or supplementary biography of a person. The use of gaiden is commonly used in popular Japanese fiction to refer to a spin-off (canonical or otherwise) of a previously published work that is neither officially considered a sequel nor a prequel. However, some gaiden are retold stories in the perspective of a different character, similar to that of a flashback.