Often it is a thin white cover that appears inside of the outer eyelids. It is a tough covering which protects eyes, especially during feeding. Many animals, such as cats, dogs, birds, reptiles, some mammals, some sharks, and even humans, have these membranes. (In the human, the nictating membrane is the apparently useless pink lump in the inner corner of the eyes.)
Nictating membranes are also popular in fictional characters such as Vulcanss.