“ISO” on cameras refers to a sensitivity rating defined by standards from the International Organization for Standardization. It is not really an expansion used in photography; ISO is the organization’s short-form name, chosen so it works across languages rather than matching initials in English or French.

Historically, film speeds were standardized under older systems such as ASA and DIN. ISO standards later unified these ideas, so camera sensitivity came to be labeled simply as “ISO.” For film, there are ISO film-speed standards; for digital cameras, there is also an ISO standard defining how digital still camera sensitivity is rated.

In practical terms, on a digital camera ISO is closely related to sensor/amplifier gain: increasing ISO makes the camera produce a brighter image from the same amount of light, but also increases visible noise. So “ISO” is the standardized sensitivity rating users see, while “gain” is the more technical concept behind it.