from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A protein of the retina, especially the protein constituent of rhodopsin, that makes up one of the visual pigments.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. Any of a group of light-sensitive proteins in the retina
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. retinal protein formed by the action of light on rhodopsin
Probably back-formation from rhodopsin.(American Heritage® Dictionary of the English Language, Fourth Edition)