Attention can be either involuntary (spontaneous) or intentional. It can be characterized by the length of time it remains active (attention span), by the strength of mental processes involved (attention intensity), and by the number of items it could be focused on (attention division). It greatly depends on an individual’s temperament, emotions, interests, well-being, age, and surroundings.
One of the first major psychologists, William James, is the author of the most famous definition of attention, which he defined in these words:

Everyone knows what attention is. It is the taking possession by the mind in clear and vivid form, of one out of what seem several simultaneously possible objects or trains of thought…It implies withdrawal from some things in order to deal effectively with others ( Jacknicke 1995).