vitamin D: a group of fat-soluble prohormones (hormone precursors), the two major forms of which include vitamin D2 and vitamin D3. Vitamin D2 is not found in animals, but is manufactured commercially and is the predominant form for prescription use in the US. Vitamin D3 is the natural-occurring form of vitamin D produced from the reaction of ultraviolet light on the skin. It has been found to be important in protecting the body from a wide range of diseases including breast cancers, cardiovascular disease, stroke, osteoporosis, osteomalacia, autoimmune diseases (such as multiple sclerosis), rheumatoid arthritis, diabetes, and schizophrenia.