Nictitating membrane and Protective fold

Nictitating membrane noun - A protective fold of skin in the eyes of reptiles and birds and some mammals.

Protective fold and nictitating membrane are semantically related. In some cases you can use "Protective fold" instead a noun phrase "Nictitating membrane".

Protective fold noun - A flap of tissue that protects what it covers.

Nictitating membrane and protective fold are semantically related. Sometimes you can replace phrase "Nictitating membrane" with "Protective fold", this phrases are similar.