The Chemical (IUPAC Naming convention): (3β,5Z,7E)-9,10-secocholesta-5,7,10(19)-trien-3-ol
You probably know it as: Vitamin D
Vitamin D is somewhat of a misnomer - it's actually not a vitamin. A vitamin is a chemical that is necessary for normal growth, development, and function that is not synthesized in the body. However, vitamin D is synthesized in the body. As shown in the diagram below, when 7-Dehydrocholesterol is irradiated with the sun's ultraviolet light (hν) one bond is broken and another is formed. Cholecalciferol is another name for vitamin D3.
So, technically it's not a vitamin. It got the name vitamin D because it was discovered while looking for an explanation of rickets - a disease that we now know is due to vitamin D deficiency. Sometimes you'll hear people talk about "absorbing vitamin D" from the sun, but that's not quite true. More correctly, vitamin D is synthesized in your skin using the sun's energy.