ETYM Ablative of Latin folium leaf. Related to Foil.
1. A leaf of a book or manuscript.
2. A sheet of paper once folded.
3. A book made of sheets of paper each folded once (four pages to the sheet); hence, a book of the largest kind.
4. The page number. The even folios are on the left-hand pages and the odd folios on the right-hand.
5. Folded sheet of paper; leaf of book, etc.; largest size of book.
6. A printed page number.

To number pages of.