- ⧽ You know, I only took *three days* [vacation] after the Aragorn/Brego slash.
- —Rhus Radicans, "Broken Doll"
Yiff is defined as pornographic material involving anthropomorphic animal characters (generally known as "furries"). This is not especially common in fanfic as few continua involve such characters. Most yiff fiction is original fic produced by the furry community. When it shows up in fanfic, it is almost invariably in fandoms where the original work is aimed at children (e.g. Disney), since they are the most likely to feature anthropomorphs.
May also be used to describe scenes involving non-anthropomorphic animals, such as those in Celebrian, though human-animal intercourse is more accurately referred to as bestiality. A key difference between the two is that furries have human sapience and can give consent; non-anthro animals usually don't and can't.
Bad yiff often includes very bad biology and usually a lot of squick, even for those who actually like furry characters. In fact, especially for those who like furry characters, because they often remember the victim characters from their childhood favourite fandoms. See Rule Thirty-Four.
Missions of fics in this category include: