supplementary biography
A biographical work that supplements a main biography (本伝). Often used in historical or literary contexts.
See also: 本伝
A spin-off of this manga was made into an anime.
This supplementary biography collects episodes not included in the main biography.
side story; spin-off; anecdote
A story that branches off from a main work, such as a novel, film, or game. Can also refer to an anecdote or a tale about a side character.
A spin-off of the popular game will be released next month.
He amusingly told anecdotes from his travels.
A side story of that series is being serialized in a magazine.
逸話 is a little-known anecdote about a person, while 外伝 can specifically mean a story about a side character branching off a main work.
外伝 (a side story or spin-off tale) is pronounced identically to 外電 but has a completely unrelated meaning about narrative content, not news reporting.
Standard kanji spelling. The reading がいでん is the only one given.
From 外 (outside, external) + 伝 (tradition, biography). The compound originally referred to a supplementary biography outside the main biographical tradition.