Quel

https://en.wikipedia.org/wiki/Prequel
https://en.wikipedia.org/wiki/Sequel
https://en.wikipedia.org/wiki/Category:Interquels

Also

1. paraquel - a story that takes place simultaneously with a certain other story
2. circumquel - a story that takes place partly before, partly after a certain other story
3. inquel - a story that takes place during a gap in some other story's narrative

Source

http://jimhenry.conlang.org/review/paraquel.htm