When the fun is deep enough, it becomes joy.
It is hard to describe joy without getting religious about it. So, I let myself get that way for a minute or several (for fun, mind you), and found this:
“It is a great mitzvah (sacred deed) to be in joy always…. When there is defect in one’s quality of joy, it leads to illness…and it is through rejoicing that all illnesses are cured…The principle of the matter is that you should muster all your powers to be always in a state of joy. For our nature is to incline toward being depressive and sullen due to afflictions that befall us and to the circumstances of the moment, and everyone is filled with troubles. Therefore, you need to move yourself with great effort towards becoming joyful always and to gladden yourself with whatever it is that you can find, and even by means that appear silly.”
- Rabbi Nachman of Breslav Likutei HaMoHaRaN Tanina, Chapter 24, paragraphs 1 and 2
More, doubtlessly, later.