vitamin Dn. A fat-soluble vitamin occurring in several forms, especially vitamin D2 or vitamin D3, that is required for normal growth of teeth and bones and is involved in many other metabolic processes, produced in the skin upon exposure to sunlight and present in fish, eggs, and fortified foods such as milk.vitamin D2n. A white crystalline compound, C28H44O, produced commercially by ultraviolet irradiation of ergosterol obtained from fungi or plants, and used as a dietary supplement. Also called ergocalciferol.vitamin D3n. A colorless crystalline compound, C27H44O, found in fish, eggs, and fortified foods and formed in the skin of humans and other vertebrates upon exposure to ultraviolet radiation, as in sunlight. It is often used as a dietary supplement. Also called cholecalciferol.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.vitamin D n, pl D vitamins (Biochemistry) any of the fat-soluble vitamins, including calciferol and cholecalciferol, occurring in fish-liver oils (esp cod-liver oil), milk, butter, and eggs: used in the treatment of rickets and osteomalaciaCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014vitamin D n. any of the several fat-soluble, antirachitic vitamins D1, D2, D3, occurring in milk and fish-liver oils, esp. cod and halibut: essential for the formation of normal bones and teeth. [1920?25] vitamin D1 n. a form of vitamin D obtained by ultraviolet irradiation of ergosterol. [1930?35] vitamin D2 n. calciferol. [1930?35] vitamin D3 n. a form of vitamin D, C27H43OH, occurring in fish-liver oils, that differs from vitamin D2 by slight structural differences in the molecule. Also called cholecalciferol. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.vitamin D Any of a group of vitamins necessary for normal bone growth. Vitamin D is found in milk, fish, and eggs and can be produced in the skin on exposure to sunlight.The American Heritage® Student Science Dictionary, Second Edition. Copyright © 2014 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.