Definitions of folio:

• noun:   a book (or manuscript) consisting of large sheets of paper folded in the middle to make two leaves or four pages

  Example: "The first folio of Shakespeare's plays"

• noun:   a sheet of any written or printed material (especially in a manuscript or book)
• noun:   the system of numbering pages
• name:  A surname (very rare: popularity rank in the U.S.: #57262)