Flow Theory

From Wikipedia

With Flow the psychologist Mihaly Csikszentmihalyi names the feeling of complete and energized focus in an activity, with a high level of enjoyment and fulfillment.As Csikszentmihalyi sees it, the components of a Flow producing activity are:

We are up to the activity.
We are able to concentrate on the activity.
The activity has clear goals.
The activity has direct feedback.
We feel that we control the activity.
Our worries and concerns disappear.
Our subjective experience of time is altered.

Not all of these components need to be present together for Flow to be experienced.

From Deepfun.com

There are many characteristic experiences that are associated with fun: the sense of timelessness, of being at one (with mind and mountain), of exhilaration, focus, immediacy. And all of these are characteristic of what we, regardless of activity, call "fun."

According to Dr. Mihaly Csikszentmihalyi's well-documented observations and research, and his wide-scale gathering of personal observations, there is pretty much universal agreement that when there isn't a high correlation between the challenge (the height of the mountain, depth of the dive) and the ability to meet that challenge, fun is something we're definitely not having. The main dialog (dynamic), according to Mihaly "just call me Mike" Csikszentmihalyi, is between Challenge and Ability. When the challenge is greater than our abilities, we become anxious and potentially dead. When the challenge is significantly less than that of which we are worthy, we become bored, and potentially dead.

Maintaining the dynamic balance between abilities and challenge is key to the fun experience in work. That is, keeping it dynamic. Making it possible for anyone to find exactly the right amount of challenge needed to engage exactly those abilities needed to access Flow.Which means that when work is fun we have created complex, but negotiable challenges, challenges that allow the individual to engage or disengage, to work harder or work safer.

So, there we have it: Fun defined as Flow, which is defined as a function of the relationship between Challenge and Ability. (see this and this, but especially this for more about the attributes of Flow). Whether or not it is true, it is definitely fun.