The experience of ‘flow’ (sometimes also called ‘optimal human experience') can arise in the context of an activity that balances skills with challenges, has clear goals and provides unambiguous and immediate feedback.
The experience of flow is characterized by total and effortless concentration on the activity, enjoyment, sense of control, merging of action and awareness, loss of sense of self-consciousness, and altered experience of time. Sometimes flow is referred to as 'being in the zone'.
Mihaly Csikszentmihalyi is the originator of this work, and you can see him talking about it below.
One of the questions my PhD will look at is whether the construct of flow is useful in understanding optimal experiences in persons with dementia. When I started out on this journey, the possibility of a link between flow and dementia was my central question but this shift is part of the story you can read about on my PhD blog felt experience.