Activities that are short in duration, interstitial and sub- ordinated within the stream of action, and often so routinized as to occur almost outside awareness.
Nakamura, J. & Csikszentmihalyi, M. (2002). The concept of flow. In Snyder, C.R. & Lopez, S.J. (Eds.). Handbook of Positive Psychology. New York; Oxford University Press, 89-105.