nictitating

Nictitating is an adjective describing anything related to the nictitating membrane, or third eyelid. The noun form nictitation (or nictitation) refers to the act of blinking or moving the membrane across the eye. The term derives from Latin nictare, meaning to wink or blink.

In many vertebrates, the nictitating membrane is a translucent or transparent third eyelid located at the inner corner of the eye. It can be drawn across the cornea by specialized muscles to protect the eye from debris, dust, and excessive light while still permitting vision through its see-through surface. It also helps distribute tears or moisture across the eye.

The membrane is especially developed in birds, reptiles, and many mammals, but its size and mobility vary widely. In some species it is a prominent, fully functional structure; in others it is small or only a vestigial fold. Aquatic or semiaquatic animals often rely on a more protective, well-developed nictitating membrane for underwater vision and eye protection during rapid movement or contact with water.

In humans, the nictitating membrane is vestigial. The plica semilunaris and associated tissue remain in the inner corner of the eye, but the membrane itself does not function as a separate protective lid in ordinary vision. The concept of nictitation is primarily used in anatomical or zoological contexts to describe the presence, form, or action of the third eyelid.