third eyelid vs protective fold

third eyelid and protective fold both are nouns.

Word	Noun	Adjective	Verb	Adverb
third eyelid	Yes	No	No	No
protective fold	Yes	No	No	No

As nouns, protective fold is a hypernym of third eyelid; that is, protective fold is a word with a broader meaning than third eyelid:

third eyelid: a protective fold of skin in the eyes of reptiles and birds and some mammals
protective fold: a flap of tissue that protects what it covers

third eyelid (noun)	protective fold (noun)
a protective fold of skin in the eyes of reptiles and birds and some mammals	a flap of tissue that protects what it covers