A film base is a transparent substrate which acts as a support medium for the photosensitive emulsion that lies atop it. Despite the numerous layers and coatings associated with the emulsion layer, the base generally accounts for the vast majority of the thickness of any given film stock. Historically there have been three major types of film base...
Flexible support on which light sensitive emulsion is coated
The flexible, usually transparent support on which photographic emulsions and magnetic coatings are carried. (Film Editing)
Flexible support on which light sensitive emulsion is coated.