Selective concentration or focus on a particular stimulus.
Attention describes the focusing of perceptive awareness on a particular stimulus or set of stimuli that results in the relative exclusion of other stimuli and is often accompanied by an increase in the readiness to receive and to respond to the stimulus or set of stimuli involved. A state of attention may be produced initially in many ways, including as a conscious, intentional decision, as a normal function of social interaction, or as a reaction to an unexpected event. In any case, attention is a fundamental component of learning. There is evidence that very young human infants have an innate ability and inclination to attend to, however briefly, particular instances of auditory or visual stimulation. Children often demonstrate the effects of their attention in the form of apparent misperceptions. For example, the relative size of objects near the center of a child's visual stimulus field is regularly overestimated by the child. In human adults, generally, attention seems to be directly related to the novelty, incongruity, complexity, or personal significance of the situation. As situations become increasingly familiar or similar to situations previously experienced by an individual, the actions of that individual become increasingly routine, and the individual becomes less attentive. There are distinct and measurable neurological and physiological, bioelectric and biochemical aspects and correlates of attention, and the capacity to achieve or to maintain a state of attention may be limited by a number of mental or physical dysfunctions.
In psychology, the term "attention span" is used technically and specifically to mean the number of separate stimulus elements, or the amount of stimulus material, that can be perceived and remembered after a brief presentation. In popular usage, the term attention span is used to mean the amount of time that can be continuously spent in a state of attention.
Hans, James. The Mysteries of Attention. Albany: SUNY Press, 1993.