"Presence: the perceptual illusion of nonmediation"
 
The term "perceptual" indicates that this phenomenon involves continuous (real time) responses of the human sensory, cognitive, and affective processing systems to objects and entities in a person's environment. An "illusion of nonmediation" occurs when a person fails to perceive or acknowledge the existence of a medium in his/her communication environment and responds as he/she would if the medium were not there. Although in one sense all of our experiences are mediated by our intrapersonal sensory and perceptual systems, "nonmediated" here is defined as experienced without human-made technology (note that under this definition even hearing aids and eyeglasses are media that "come between" our environment and our perceptual system).
 
The illusion of nonmediation can occur in two distinct ways: (a) the medium can appear to be invisible or transparent and function as would a large open window, with the medium user and the medium content (objects and entities) sharing the same physical environment; and (b) the medium can appear to be transformed into something other than a medium, a social entity.
Presence in this view can not occur unless a person is using a medium. It does not occur in degrees but either does or does not occur at any instant during media use; the subjective feeling that a medium or media-use experience produces a greater or lesser sense of presence is attributable to there being a greater or lesser number of instants during the experience in which the illusion of nonmediation occurs.
It should be noted that this illusion does not represent a perceptual or psychological malfunction or psychosis, in which the mediated experience is consciously confused with what is nonmediated or "real." Clearly when asked, users of any current or likely future medium can accurately report that they are using a medium (the "holodeck" in the "Star Trek" television series and films is a exception; see in particular the episode of "Star Trek: The Next Generation" titled "Ship in a Bottle").
 
This definition of presence can be applied to any medium and encompasses each of the six conceptualizations discussed above. A medium that becomes invisible and produces a perceptual illusion of nonmediation analogous to an open window can provide rich verbal and nonverbal information for social interaction (presence as social richness); objects and entities in such a medium should appear perceptually (if not socially) vivid and real (presence as realism); the illusion that there is no medium at work means there is no border between "this side" and "the other side" of the medium, so users can perceive that they have moved to the other side, that objects/entities from the other side have entered their immediate environment, or that they and other users are sharing a real or artificial environment (presence as transportation); the illusion of nonmediation will be more complete if the medium is perceptually and psychologically immersive (presence as immersion); and if we encounter people or entities within such a medium, even if there is no possibility of true social interaction with them, we are encouraged to respond to social cues they provide just as we would in nonmediated communication (presence as social actor within medium). Finally, when the medium itself presents us with social cues normally reserved for human-human interaction we are likely to perceive it not as a medium but as an independent social entity, a tranformed medium (presence as medium as social actor).
 
Because it is a perceptual illusion, presence is a property of a person. However it results from an interaction among formal and content characteristics of a medium and characteristics of the media user, and therefore it can and does vary across individuals and across time for the same individual. We turn next to the limited evidence, as well as the speculation, concerning which of these characteristics encourage and discourage a sense of presence in media users, and the effects of presence.
 
uit: At the Heart of It All: The Concept of Presence, Matthew Lombard
Theresa Ditton, 1997
Department of Broadcasting, Telecommunications, & Mass Media Temple
University