The holistic sensation that people feel when they act with total involvement. When one is in flow, he/she experiences it as a unified flowing from one moment to the next, in which he/she is in control of his actions, and in which there is little distinction between self and environment, between stimulus and response, or between past, present, and future. Flow is an autotelic experience, in that it has no external goals or external rewards; people seek flow primarily for itself, not for the incidental extrinsic rewards that may accrue from it.
Csikszentmihalyi, M. (2000). Beyond Boredom and Anxiety. San Francisco: Jossey-Bass.