(a Japanese-language word often translated as "side story" or "tale") is used to refer to an anecdote or supplementary biography of a person. The term gaiden is commonly used in popular Japanese fiction to refer to a spin-off (canonical or otherwise) of a previously published work that is not 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.

Works that includes the use of the word gaiden in their titles include Fire Emblem Gaiden, Kakashi Gaiden (a set of chapters of the Naruto manga which tells the story of the "Great Ninja War", an event occurring decades before the actual series), Ten no Haō Hokuto no Ken Raō Gaiden (spin-off of Fist of the North Star, which is told through the eyes of Kenshiro's brother, Raoh), Gensō Suikogaiden (spin-off of Suikoden), Albert Odyssey Gaiden (released in English as Albert Odyssey: Legend of Eldean) and Seiken Densetsu: Final Fantasy Gaiden (released in North America as Final Fantasy Adventure and in the PAL region as Mystic Quest). In addition, the word gaiden has been used in English titles as well such as Ninja Gaiden (story told in the super ninja, Ryu Hayabusa's perspective) (the American title for the Japanese series Ninja Ryūkenden) and Resident Evil Gaiden (a European-developed spin-off of the Japanese Resident Evil franchise) as well as Super Robot Taisen ALpha Gaiden, which tells a future that would happen without the heroes fighting at the time of need.