Source : Webster's Revised Unabridged Dictionary (1913)

Fol'io \Fol'io\, v. t.
   To put a serial number on each folio or page of (a book); to
   page.

Fol'io \Fol'io\, a.
   Formed of sheets each folded once, making two leaves, or four
   pages; as, a folio volume. See {Folio}, n., 3.