The abbreviation "IU" on vitamin labels means "international units." It is a standard used by the scientific community to measure the amount of fat-soluble vitamins such as A, D and E in a capsule or tablet.
Rather than measuring the mass or volume of a specific vitamin, IU measures the potency and the biological effect. This means that 1 IU of vitamin A can be very different from 1 IU of vitamin D or E. The international unit is used to compare one biologically active substance, such as a vitamin, with another one that has the same or similar properties.
