n. & adj.
n. (pl. -os) 1 a leaf of paper etc., esp. one numbered only on the front. 2 a leaf-number of a book. 3 a sheet of paper folded once making two leaves of a book. 4 a book made of such sheets.
adj. (of a book) made of folios, of the largest size. in folio made of folios.
[ L, ablat. of folium leaf, = on leaf (as specified) ]