(from Latin:  folium, “leaf).  A term used to describe the leaves of a manuscript book (codex).  By convention, manuscripts are numbered by folios, rather than pages.  Each leaf is identified by a number, and the front and the back of the leaf are distinguished by the terms recto (for the front) and verso (for the back). Web resource here.

Provinces Bringing Tribute (folio 23 verso) and Portrait of Otto III (folio 24 recto) from the Gospels of Otto III.  Ink, paint, gold on parchment.  Ca. 1000.  Munich, Bayerische Stattsbibliothek, Munich, Clm. 4453.