A measure of the speed of the photographic emulsion.
The scale is arbitrary, but the important thing to know is that the speed at which the image registers on the film is proportional to the ASA number. Thus ASA 400 film registers an image twice as fast as ASA 200 film and four times as fast as ASA 100 film. The ASA number is now combined with the European DIN rating as a composite ISO rating.
ASA 400 film is marked ISO 400/27°, where 27 is the DIN rating corresponding to ASA 400.