Home›Browse›T›third eyelidthird eyelidOur editors are putting together more complete content — check back in about 20 minutesthird eyelidthird eyelid — nounnoun1. a transparent membrane that moves across the eye to protect and moisten it1.nounDefinitiona transparent membrane that moves across the eye to protect and moisten itExampleCats have a third eyelid that helps keep their eyes clean.