To me, it refers to something that gives me pleasure, that I could do for long periods of time without getting tired of it. 'Fun' activities are those that have me so immersed in it that I lose track of time.
What's your personal definition of fun?
For me, the things I consider 'fun' are not stereotypical. Lots of people see 'fun' as something similar to a wild party or a carnival. For me, it's the quieter things, like reading a really good book, or going on this forum.