Fun makes me smile, I think that is a good general way to describe the word to me. Going into a mob and blowing it up is fun, this makes me smile. Barely surviving a +3x8 mob of Carnies is fun. Playing Hades puts a big ass smile on my face. In most cases I see fun and joy as two sides of the same coin.
Fun by itself isn't enough to keep me engaged for too long though. I need something to work toward. First it was the soft cap, then soloing various TF, another build, X amount of influence, etc. For AC Odyssey I wanted to get every vantage point, Hades all the achievements, etc. Mix in fun of course, but I personally like that long term goal that requires repetition and can sometimes be tedious.