nictitating membrane

noun

1. a thin membrane, or inner or third eyelid, present in many animals, capable of being drawn across the eyeball, as for protection.

noun

1. (in reptiles, birds, and some mammals) a thin fold of skin beneath the eyelid that can be drawn across the eyeAlso called: third eyelid, haw

1. A transparent inner eyelid in birds, reptiles, amphibians, and some mammals that protects and moistens the eye without blocking vision.