The term photographic grain has been used in the jargon of the trade with three separate meanings: it applies sometimes to the grainy pattern visible in a highly enlarged photograph, sometimes to a particle of the developed silver in the image, and other times to the undeveloped silver halide particles of the original film. A review of some significant topics related to each of these three meanings is given, emphasizing the interactions between the three concepts.
J. F. Hamilton, "The Photographic Grain," Appl. Opt. 11, 13-21 (1972)