Psychologist Mihály Csikszentmihályi has discussed this phenomenon in his book Flow: The Psychology of Optimal Experience, defining it as a flow state of mind in which people are completely engrossed in an activity, especially if the activity engages their creative abilities. During this optimal experience they feel empowered, alert but unaware of themselves, and at the peak of their abilities. It is, therefore, a state of mind that a person reaches by doing an activity when they are totally focused on while enjoying it.
 
Among the characteristics experienced during the flow state are the forgetfulness of worries and the feeling of being challenged but not overwhelmed, with the ability to finish the task at hand.
We also talk about it in our blog:
The Zone
The Zone

We admire how our elite athletes reach their maximum potential, achieving a state of absolute concentration necessary to achieve their goals and enjoy the moment during the competition. How do they do it? The secret to the maximum performance level is in The Zone... now you too can know how to enter and experience this state of emotional satisfaction.