A
reflective layer of
cells behind the
retina of the
eye, present in many
mammals. Familiar animals with a tapidum lucidum include
cats and
dogs.
Photons (light particles) sometimes aren't absorbed by the retina; instead they go right through, as it is a very thin layer of cells. The tapidum lucidum reflects the light back through the retina so it has a chance to be detected again. This greatly increases the light sensitivity of the eyes.
The tapidum lucidum is also responsible for the well-known myth that cats' eyes glow in the dark. The truth is you are simply seeing ambient light reflecting off this reflective layer.
Unfortunately, humans didn't evolve a tapidum lucidum, so while our cats are effortly bounding about in the dark, we are tripping over things and walking into walls. The fact that we are diurnal mammals probably contributed to this.