What is the purpose of a cat’s third eyelid?

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A Maine Coon longhair cat with cream and white fur sits beside a tabby shorthair cat in a garden with green foliage in the background. — cat Two kinds of domestic cats.

The third eyelid, scientifically known as the nictitating membrane, serves as a protective layer for a cat's eye.

This thin, translucent membrane is located at the inner corner of the eye. It can extend across the eyeball to provide an extra barrier against debris, injury, or irritation, while still allowing the cat to maintain some level of vision. In addition to protection, it helps distribute tears across the surface of the eye to keep it moist and healthy.1