The iodine number is a measure for the amount of unsaturated compounds in a fluid. A low iodine number indicates that the fluid contains few unsaturated compounds and is therefore significantly more stable when exposed to periods of oxidation than a fluid with a higher iodine number.
Oils are described as "saturated" when their iodine number is <10. However, unsaturated fluids have a much higher iodine number. Typical rapeseed oils, for example, have an iodine number of approximately 120.