General sensitivity or speed.

Just as the term chromatic sensitivity characterises the sensitivity of a film to different coloured wavelengths of light so general sensitivity, also called speed, defines its total general sensitivity to light. Speed is a measure of the exposure necessary to obtain a specific density of image on the film.
Speed values are defined by national and international standards and three are widely used today. All films produced today are given ASA (American Standard Association), DIN (Deutsche Industrie Norm), or ISO [International Standards Organisation].
High speeds or "faster" films are required for in the camera negative, so that it is possible to work with less light or in difficult conditions on the set. The speed indicated on the film is an only nominal and is defined for a particular set of development conditions. In addition, emulsions may vary slightly from one manufacturing period to the next to another. The higher the speed generally the lower the definition and the more visually obvious the "grain" structure so that there are optimum speeds for each purpose. High-speed films are not necessary for laboratory printing processes (making positives, duplicates or copies), where the various exposures can be controlled more easily. For these films, the correct exposure is very important and each type of film has to be tested in the laboratory in order to estimate the ideal exposure empirically.