n.
Pronunciation: r ō - ' däp-s ə n
Function: noun
Etymology: International Scientific Vocabulary rhod- + Greek ops is sight, vision + International Scientific Vocabulary 1 -in ― more at OPTIC
Date: 1886
: a red photosensitive pigment in the retinal rods of the eye of most vertebrates that is important in vision in dim light ― called also visual purple