membrane vs flat solid

membrane and flat solid both are nouns.

Word	Noun	Adjective	Verb	Adverb
membrane	Yes	No	No	No
flat solid	Yes	No	No	No

As nouns, flat solid is a hypernym of membrane; that is, flat solid is a word with a broader meaning than membrane:

Other hypernyms of membrane include sheet.

membrane (noun)	flat solid (noun)
a thin pliable sheet of material	a flat artifact that is thin relative to its length and width
a pliable sheet of tissue that covers or lines or connects the organs or cells of animals or plants