the numerical exposure index of a photographic film under the system adopted by the International Standardization Organization, used to indicate the light sensitivity of the film's emulsion.
iso-
a combining form meaning “equal,” used in the formation of compound words: isochromatic; in chemistry, used in the names of substances which are isomeric with the substance denoted by the base word: isocyanic acid.
ISO (ī'ěs-ō') n. An organization, the International Organization for Standardization, that sets standards in many businesses and technologies, including computing and communications.
International Organization for Standardization (technically not an initialismâterm is based on the prefix iso â, from the Greek isos, meaning âequalâ)
[ahy-soh] 
soʊ