The unit for measuring biological activity is commonly referred to as the International Unit (IU). This unit quantifies the biological effect or activity of a substance, such as vitamins or enzymes, based on its measurable impact rather than its chemical amount. For instance, when discussing vitamins, one IU corresponds to the biological activity of a set quantity of that vitamin. This unit provides a standardized method to compare the effectiveness of different substances across biological systems and applications, ensuring consistency in research and medicine.