7-dehydrocholesterol

[-dē-hī′drō-kə-lĕstə-rôl′, -rōl′]
n.
  1. A provitamin present in the skin of humans as well as the milk of mammals that becomes vitamin D3 when exposed to ultraviolet light.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.