third eyelid vs nictitating membrane

third eyelid and nictitating membrane both are nouns.

Word	Noun	Adjective	Verb	Adverb
third eyelid	Yes	No	No	No
nictitating membrane	Yes	No	No	No

As nouns, third eyelid and nictitating membrane are synonyms defined as:

third eyelid and nictitating membrane: a protective fold of skin in the eyes of reptiles and birds and some mammals

third eyelid (noun)	nictitating membrane (noun)
a protective fold of skin in the eyes of reptiles and birds and some mammals	a protective fold of skin in the eyes of reptiles and birds and some mammals