A whitish or translucent membrane that forms an inner eyelid in birds, reptiles, and some mammals. It can be drawn across the eye to protect it from dust and keep it moist. Also called third eyelid.
- Crocodilians' eyes are immobile spheres covered by three eyelids: the third eyelid, the nictitating membrane, is transparent, but protects the eye from water.
- They have a full inner eyelid, or nictitating membrane, which protects the eye from damage and drying.
- When some kingfishers dive for fish, their eyes are covered by a nictitating membrane.
Early 18th century: nictitating based on medieval Latin nictitat- 'blinked', frequentative of nictare.
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