The story
From vitamin + D (“4th letter”). From being the fourth vitamin discovered. Coined in 1922 by Elmer McCollum, who discovered the vitamin.
Etymology from Wiktionary, CC BY-SA 4.0.
vitamin D
Today it means: Any of a number of fat-soluble vitamins, required for normal bone development and preventing rickets, that can be manufactured in the skin on exposure to sunlight. Full definition →