The expressive and/or informative value with which a sound enriches a given image, so as to create the definite impression (either immediate or remembered) that this meaning emanates “naturally” from the image itself.
(Source : Chion, Michel. (1994) Audio-Vision : Sound on Screen. (Trans. Claudia Gorbman) Columbia University Press.)
Bibliography: Russo, Marco (2004). Virtual space and cinema: the front-back presentation of sound