Nictitating Membrane: A transparent inner eyelid in birds, reptiles, and some mammals that closes to protect and moisten the eye. Also called third eyelid.
Nocturnal: Of, relating to, or occurring in the night. See also Crepuscular ...
The part of the lining of the eye that covers the cornea when the eyelids close
The third eye lid in the inner angle of the eye, which may spread diagonally over the eye ball to protect the eye.
When cats are sick, one can see the third eye lid quite often, which is called prolaps.
Nictitating membrane- the correct name for the thin opaque membrane that covers the front fo the eyebal under the eyelid.
Nocturnal: Active at night
NZCF: New Zealand Cat Fancy ...
When it does occur, it involves eversion or prolapse of the gland of one or both of the cat’s third eyelids (also called the nictitating membranes). Those glands are intimately associated with and located underneath the third eyelid, and usually they can’t be seen.
Cats have a third eyelid, the ~, which is a thin cover that closes from the side and appears when the cat's eyelid opens. This membrane partially closes if the cat is sick, although in a sleepy state this membrane is often visible.
Third eyelid: The eyelid that is attached to the inner corner of the eye and extends out to the side of the eye when the animal blinks; also called the "~"
Causes of red eyes in dogs and cats ...
See also: What is the meaning of Coat, Veterinarian, Behavior, Hereditary, Chocolate?