What does research have to say about defining "fun"--what it is or when it happens? I've found some research on whether or not people rate specific things as fun, and lots of misleading titles with the word "fun" in them, but very little on what fun is (cognitively, neurologically, affectively, socially) and when it happens. I also found a couple of cogsci.SE questions but the answers are either entirely unsupported or stray into other waters (motivation, addiction).
Is there a cognitive or scientific definition of "fun"?
Is there a body of research on when fun happens?
Oddly enough, the place where you find the most information about this topic is where there's a lot of money to be made from it. Facebook games or games in general. If you focus on "why do people like to play some games and not others", then you're asking more of an economical question, and there will be a lot more information on the topic.
Example: Funology: From Usability to Enjoyment (Human-Computer Interaction Series)
There's plenty of other works of this kind.
If you want things that are not related to games, I recommend Bruce C. Daniels's Puritans At Play: Leisure and Recreation in Early New England.
I believe both of these are available on Open Library.