vitamin D

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A colorless crystalline compound, C₂₇H₄₄O, found in fish-liver oils, irradiated milk, and all irradiated animal foodstuffs. It has essentially the same biological activity as vitamin D₂.
• noun A fat-soluble vitamin occurring in several forms, especially vitamin D₂ or vitamin D₃, required for normal growth of teeth and bones, and produced in general by ultraviolet irradiation of sterols found in milk, fish, and eggs.
• noun A white crystalline compound, C₂₈H₄₄O, produced by ultraviolet irradiation of ergosterol.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Either of a number of fat-soluble vitamins, required for normal bone development and that prevents rickets; can be manufactured in the skin on exposure to sunlight.