Either of two fat-soluble sterol-like compounds, ergocalciferol (vitamin D₂) and activated cholecalciferol (vitamin D₃). Vitamin D₂ is formed from the irradiation of ergosterol, a plant sterol. However, vitamin D₃ is normally manufactured in the skin, where ultraviolet light activates the compound 7-dehydrocholesterol. Vitamins D₃ and D₂ are about equal in activity in all mammals except New World monkeys and birds, in which vitamin D₂ is approximately one-tenth as active as vitamin D₃.