The method of film speed (sensitivity) classification developed for the American Standards Association (ASA) and finally adopted by the International Standards Organization (ISO).
The scale is such, that if the ISO speed number of the film is doubled, the sensitivity of the film is doubled, e.g. if for a given lighting situation, you change from ISO 100 film to ISO 200 film, you must either close down the lens aperture by one stop, or double the shutter speed.