fo·li·o / ˈfōlēˌō/ • n. (pl. -os) an individual leaf of paper or parchment, numbered on the recto or front side only, occurring either loose as one of a series or forming part of a bound volume. ∎ Printing the leaf number in a printed book. ∎ a sheet of paper folded once to form two leaves (four pages) of a book. ∎ a size of book made up of such sheets: copies in folio. ∎ a book or manuscript made up of sheets of paper folded in such a way; a volume of the largest standard size: old vellum-bound folios | [as adj.] a folio volume.