Web fiction
Fanon
Fandom's unofficial canon: ideas from fan works so widely adopted by a fan community that they are treated as established fact.
Fanon is a portmanteau of "fan" and "canon." Wikipedia's article on fan fiction's relationship to canon notes that "certain ideas may become influential or widely accepted within fan communities": plotlines "reinvented so many times and by so many people" take on "the status of fan-produced canon." Fan fiction itself is never canonical, but a detail the fandom keeps reusing, a character's middle name, the shape of an unshown backstory, hardens into something readers expect to be true.
Fanon sits between canon, the official and authoritative works, and headcanon, one fan's private interpretation: fanon is the collective layer. Writers who came up through fan fiction should check which "facts" they know are canon and which are fanon before they shape an original story. Serial authors meet fanon from the other side: long-running stories grow fanon around every gap, and a chapter that contradicts a beloved fanon belief will hear about it in the comments.