noun
: any or all of several fat-soluble antirachitic vitamins that are related chemically to the steroids, that are essential for normal bone and tooth structure, and that occur especially in the liver oils of various fishes, in egg yolk, and in milk or are produced by activation of sterols (as by ultraviolet irradiation of individual sterols or of foods containing them): as
a. or vitamin D 2 : a crystalline unsaturated alcohol C 28 H 43 OH that is usually prepared by irradiation of ergosterol and is used as a dietary supplement in human and animal nutrition (as in fortified milk or butter) and medicinally in the treatment and control of rickets, osteomalacia, and other hypocalcemic disorders — called also calciferol, ergocalciferol ; compare viosterol
b. or vitamin D 3 : a crystalline unsaturated alcohol C 27 H 43 OH that is the predominating form of vitamin D in most fish-liver oils and is formed in the skin of animals on exposure to sunlight or ultra-violet rays and is usually made commercially by irradiation of dehydrocholesterol and that is used similarly to vitamin D 2 but is preferred for addition to poultry feeds because of its greater activity — called also cholecalciferol
c. or vitamin D 4 : a crystalline alcohol C 28 H 45 OH isomeric with dihydrotachysterol and obtained by irradiation of a dihydro derivative of ergosterol