A complex chemical used for colouring by molecular absorption (as opposed to a pigment which colours by coating the substance with solid colouring matter). 1
Kodak says that in photography it is the result of colour processing in which the silver grains or incorporated colour couplers have been converted into the appropriate dye to form part of the colour image.
Harold Brown2 believes that a ‘vital difference between a dye and a pigment is that a dye transmits light, while a pigment reflects it, both selectively.’
1 1969, The Focal Encyclopedia of Film and Television Techniques, Focal Press, London, New York
2 1967, Brown, Harold G., Notes on film identification by the examination of copies Harold G. Brown, Film Conservation Officer, National Film Archive, London.