Daydreaming is the stream of consciousness that detaches from current, external tasks when attention drifts to a more personal and internal direction. This phenomenon is common in people's daily life shown by a large-scale study in which participants spend 47% of their waking time on average on daydreaming. There are various names of this phenomenon including mind wandering, fantasy, spontaneous thoughts, etc. Daydreaming is the term used by Jerome L. Singer whose research laid the foundation for nearly all the subsequent research today. The terminologies assigned by researchers today puts challenges on identifying the common features of daydreaming, and on building collective work among researchers.

There are many types of daydreams, and there is no consistent definition among psychologists. However, the characteristic that is common to all forms of daydreaming meets the criteria for mild dissociation. Also, the impacts of different types of daydreams are not identical. While some are disruptive and deleterious, others may be beneficial in some way.