The third eyelid, also known as the nictitating membrane, is a thin, transparent eyelid that serves to protect and moisten the eye. It is found in many animals, including birds, reptiles, fish, and some mammals. In some cases, the third eyelid may be visible even when the animal's eyes are open, and it may even be used to sweep across the surface of the eye to remove foreign particles. Other synonyms for the third eyelid include the membrana nictitans, the haw, the inner eyelid, and the palpebra tertia. Understanding the different names for this unique and important part of the eye can increase one's appreciation for the complex and varied biology of the animal kingdom.