Definition: Noun. An inner, third eyelid, which serves to protect the eye from dryness and/or damage. When a cat is sick, the third eyelid will partially close, which is a signal to get him to the vet immediately, especially if other symptoms are present. Curiously enough, a very happy, drowsy, cat will sometimes show that nictating membrane.
Also Known As: third eyelid, haw
Alternate Spellings: nictitating membrane
We knew Bubba was in trouble when he became lethargic and his nictating membrane was exposed.