Flow is a state of mind when one is 'in the zone' - completely focused and in peace performing some task as results and creativity flow freely and abundantly.
Flow state percentage is the fraction of any given day that one spends in the state of flow.
Mihalyi Csikszentmihalyi is the seminal thought leader on the topic. 
- 60 bits per second is human capacity
- Can't create something new without 10 years of training? But how about generic training, that allows your next mastery to come quicker?
Talks Link to Happy Planet Index