Opsin

Part of speech: noun

Definitions

1. A class of proteins that detect light and initiate the visual response in organisms
2. Proteins found in visual cells that react to light by triggering signals to the brain
3. Molecules in the retina that absorb light and start the process of seeing

Etymology: Opsin is a scientific coinage, and its name is a clipped version of a longer word. Chemists first named the light-sensitive pigment of the eye "rhodopsin," combining the Greek "rhodon," meaning rose, with "opsis," meaning sight, since the pigment has a reddish colour. The protein part of that pigment was later given a shorter name, and "opsin" is simply the piece left over after the "rhodo" was dropped. The root "opsis" comes from Ancient Greek, where it meant "sight," "appearance," or "the power of seeing," and it is the same root that gives English words like "optic" and "optical." The ending "-in" is the standard chemical suffix used for proteins and other biological substances, so the word reads as "the sight protein." Because the word was made from classical roots to name a molecule, no ancient Greek writer ever used it. Its meaning is fixed by modern biochemistry: opsins are the proteins that absorb light inside photoreceptor cells and set off the signal that becomes vision.