What is the name of the transparent third eyelid in amphibians?

The transparent third eyelid in amphibians is called the nictitating membrane. It is a thin, movable sheet that sweeps horizontally across the eye, helping protect and moisten its surface while still allowing the animal to see.

Most frogs have a fully developed nictitating membrane, although a few aquatic anurans, including Hymenochirus, Pseudhymenochirus, and Pipa species, are exceptions. The membrane is especially useful for frogs that move between water and land: it can shield the eye underwater, during feeding, or when debris and vegetation might cause injury.

The name comes from the Latin nictare, meaning “to blink.” It is also called a third eyelid, haw, membrana nictitans, or palpebra tertia. Humans do not have a functional version, but the small fold in the inner corner of the human eye—the plica semilunaris—is considered a reduced remnant of this structure.