What does iu mean in measurement?

IU= international unit. This is a unit used to measure the activity (that is, the effect) of many vitamins and drugs. For each substance to which this unit applies, there is an international agreement specifying the biological effect expected with a dose of 1 IU. Other quantities of the substance are then expressed as multiples of this standard. This also means that this measurment is not based on sheer volume or weight of the substance, but rather the effect.