Earlier in the film age, the sensitivity was defined as ASA In North America – ASA (American Standards Association)
ASA was a unit of measurements of film speed or sensitivity to light before ISO officially arrived. ASA is a scale created by the American Standards Association, but it is no longer widely used.
Definition ASA in film time which is similar to ISO – ASA is the rating of sensitivity to light for chemical-based, emulsion film stocks used in motion picture film cameras. Higher numbers represent film emulsion that is more sensitive to light. Lower numbers represent less sensitive emulsion. The most common average range of ASA sensitivity used is 100, 200, 400, 800.
In the Film age the ISO was known as ASA in North America
And different parts of the world used varying systems ranging from DIN (Deutsches Institut für Normung) for most of Europe to BSI (British Standards Institution) in the U.K. and GOST (Cyrillic: ГОСТ) in the former Soviet Union and several others systems as well.
In the end, it was the ASA system (which is arithmetic based) that won out over the others, most of which were logarithm based, DIN probably being the most widely used of these.
From Mid 1979 ASA Changed to ISO 12232
Both of these (ASA & DIN) were in use until as recently as the mid-1979, when the International Organization for Standardization (ISO) taken over, and added their own initial set of definitions.
Now ASA was renamed as ISO 12232 by International Organization for Standardization
Today’s ISO values are defined by ISO 12232:2019 (that is, ISO standard number 12232, last revised in 2019). This ISO standard “specifies the method for assigning and reporting ISO speed ratings, ISO speed latitude ratings, standard output sensitivity values, and recommended exposure index values, for digital still cameras. ISO 12232:2019 is applicable to both monochrome and color digital still cameras
So, the short answer is the ASA is re-defined ISO 12232 ( International Organization for Standardization) ,
We can now say the ISO stands for International Organization for Standardization 12232.2019
ISO 12232 definition is updates with time to time
UPDATE LOG OF ISO 12232