Transparent, flexible plastic material, usually of cellulose acetate or polyester, on which a light-sensitive emulsion is coated, or on which an image can be formed by various transfer processes, used for still photography and cinematography. Also, a thin coating of collodion or gelatin spread on photographic paper or plates, or used by itself instead of a plate.

Source: Art & Architecture Thesaurus

http://vocab.getty.edu/page/aat/300014638