nictitating membrane In some vertebrates, a lid-like membrane that is attached to the anterior corner of the eye. When drawn across the eye it reduces illumination of the retina. It is common in Reptilia and Aves, and present in some Mammalia and in many sharks (e.g. those of the family Carcharhinidae).

nic·ti·tat·ing mem·brane / ˈniktiˌtāting/ • n. Zool. a whitish or translucent membrane that forms an inner eyelid in birds, reptiles, and some mammals. It can be drawn across the eye to protect it from dust and keep it moist. Also called third eyelid.

nictitating membrane A clear membrane forming a third eyelid in amphibians, reptiles, birds, and some mammals (but not humans). It can be drawn across the cornea independently of the other eyelids, thus clearing the eye surface and giving added protection without interrupting the continuity of vision.