flyleaf vs folio

flyleaf and folio both are nouns.

Word	Noun	Adjective	Verb	Adverb
flyleaf	Yes	No	No	No
folio	Yes	No	No	No

As nouns, folio is a hypernym of flyleaf; that is, folio is a word with a broader meaning than flyleaf:

flyleaf: a blank leaf in the front or back of a book
folio: a sheet of any written or printed material (especially in a manuscript or book)

Other hypernyms of flyleaf include leaf.

flyleaf (noun)	folio (noun)
a blank leaf in the front or back of a book	a book (or manuscript) consisting of large sheets of paper folded in the middle to make two leaves or four pages
	a sheet of any written or printed material (especially in a manuscript or book)
	the system of numbering pages