Milk that has been exposed to ultraviolet light to increase its vitamin D content. UV irradiation converts naturally occurring sterols in the milk, particularly ergosterol, into vitamin D, enhancing the milk’s nutritional value without additives. The technique was developed in the early twentieth century as a practical response to rickets, the bone-deforming disease caused by vitamin D deficiency in children. Although direct fortification with vitamin D3 later became more common and easier to standardise, irradiated milk played an important historical role in public-health nutrition. The process is distinct from ionising food irradiation: UV light does not penetrate deeply and leaves the milk’s safety and flavour essentially unchanged.