Glossary
Third eyelid
A thin fold of tissue at the inner corner of a cat's eye that sweeps across the surface to protect and moisten it.
Also called: nictitating membrane, haw, inner eyelid
Cats have three eyelids. Besides the upper and lower lids, a third eyelid, also called the nictitating membrane or haw, sits at the inner corner of each eye. It slides across the eye from the side, spreading tear film, clearing dust and acting as a shield, for example during a scuffle or while hunting in long grass.
Most of the time it stays tucked away and you barely see it. A sleepy or very relaxed cat may show a little of it for a short while, and a drowsy kitten often does. That is normal.
A third eyelid that stays visible when the cat is wide awake needs attention, especially when it shows in both eyes or comes with discharge, squinting, a sunken eye, vomiting, diarrhoea or weight loss. It can point to pain in the eye, nerve problems, dehydration, illness elsewhere in the body or loss of weight around the eye. It cannot be judged from home, so ask your vet to examine the cat rather than waiting for it to resolve.
Last reviewed November 7, 2026