Definition of third eyelid in English:

third eyelid

noun

informal
  • another term for nictitating membrane
    • ‘Eversion of the nictitating membrane is a condition in which the third eyelid protrudes.’
    • ‘Crocodilians' eyes are immobile spheres covered by three eyelids: the third eyelid, the nictitating membrane, is transparent, but protects the eye from water.’
    • ‘Later, she had a real breakthrough when she discovered that prions collect in pockets of lymphoid tissue in a sheep's nictitating membrane, or third eyelid.’
    • ‘These membranes, essentially a third eyelid, can cover the exposed portion of the eye, since, as in all sharks, the upper and lower eyelids of catsharks cannot completely cover the eyeball.’
    • ‘A crocodile's third eyelid flushes tears across the eyeball to lubricate its passage and to clean and protect the eye.’