A third eyelid, also known as a haw or nictitating membrane, is a structure found in the eyes of reptiles, birds, and some mammals. It consists of a transparent membrane which is drawn across the eye. The third eyelid lies underneath the two outer eyelids, and it serves a number of different functions in the eye. People who have been around cats and dogs may have seen the third eyelid, as these mammals possess a nictitating membrane which is sometimes visible during sleep.

Like the outer eyelids, the third eyelid is designed to protect the eye from damage. It can be used to sweep dust and other debris from the surface of the eye, and to moisten it. One of the glands of the eye is located at the base of the third eyelid, providing a steady source of fluid to keep the eye moist so that it will function well. Many animals that live in dusty or dirty environments may utilize their third eyelids periodically to blink away debris.

In some diving animals, this structure is used to cover the eye while swimming so that the animal can see, and its eyes are protected from debris and other substances in the water. Birds utilize the third eyelid for protection while feeding their young, ensuring that the baby birds do not accidentally peck the eyes of their parents in their hurry to eat. Creatures like sharks may employ their third eyelids when they go on the attack, to protect their eyes from thrashing or panicking prey.

Humans and most primates lack a third eyelid, although some vestigial structures around the eye seem to suggest that humans once possessed this anatomical feature. People who are curious to see this membrane in action may be able to find an obliging cat or dog. In these animals, the nictitating membrane sometimes shows during sleep, or when the animal is woken suddenly from a nap. Chickens and other domestic poultry may also display their third eyelids if they are held and gently stroked or rocked to calm them.

Animals with third eyelids are subject to some disorders which can affect the nictitating membrane, causing a variety of problems. Sometimes the membrane rolls or inverts so that it cannot be closed, forming a small mass in the corner of the eye which can lead to irritation. Animals may also sometimes have difficulty closing or opening their third eyelids, and in some cases, the gland at the base of the third eyelid may move, causing a condition known as cherry eye. Veterinary surgery is generally required to correct these problems.