Dictionary.com
definitions

nictitating membrane

noun
  1. a thin membrane, or inner or third eyelid, present in many animals, capable of being drawn across the eyeball, as for protection.

Origin of nictitating membrane

1705–15; nictate + -ing2
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for nictitating membrane

nictitating membrane

noun
  1. (in reptiles, birds, and some mammals) a thin fold of skin beneath the eyelid that can be drawn across the eyeAlso called: third eyelid, haw
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
nictitating membrane in Science

nictitating membrane

[nĭk′tĭ-tā′tĭng]
  1. A transparent inner eyelid in birds, reptiles, amphibians, and some mammals that protects and moistens the eye without blocking vision.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.