Веселье. It's a bit closer to "joy" or "merry-ness", though. Why?
Lots of other words seem to be used in similar contexts, e.g. 'prikol,' 'klyevo', maybe even 'pizdyetz' (some may be archaic, it's been a while since I had been immersed in Russian), but none of them seem to be exactly right. I think it's weird that there is no exact isomorphism from such a basic English concept.
Nobody uses the word Веселье in colloquial Russian in this sense, but people use "fun" in colloquial English all the time.
Another month, another rationality quotes thread. The rules are: