The Collaborative International Dictionary of English v.0.48:

Nictitate \Nic"ti*tate\, v. i. [See Nictate.]
   To wink; to nictate.
   [1913 Webster]

   Nictitating membrane (Anat.), a thin membrane, found in
      many animals at the inner angle, or beneath the lower lid,
      of the eye, and capable of being drawn across the eyeball;
      the third eyelid; the haw.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nictitating membrane
    n 1: a protective fold of skin in the eyes of reptiles and birds
         and some mammals [syn: nictitating membrane, third
         eyelid]