The Collaborative International Dictionary of English v.0.48:

Folio \Fol"io\, a.
   Formed of sheets each folded once, making two leaves, or four
   pages; as, a folio volume. See Folio, n., 3.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Folio \Fol"io\, n.; pl. Folios. [Ablative of L. folium leaf.
   See 4th Foil.]
   1. A leaf of a book or manuscript.
      [1913 Webster]

   2. A sheet of paper once folded.
      [1913 Webster]

   3. A book made of sheets of paper each folded once (four
      pages to the sheet); hence, a book of the largest kind.
      See Note under Paper.
      [1913 Webster]

   4. (Print.) The page number. The even folios are on the
      left-hand pages and the odd folios on the right-hand.
      [1913 Webster]

   5. A page of a book; (Bookkeeping) a page in an account book;
      sometimes, two opposite pages bearing the same serial
      number.
      [1913 Webster]

   6. (Law) A leaf containing a certain number of words, hence,
      a certain number of words in a writing, as in England, in
      law proceedings 72, and in chancery, 90; in New York, 100
      words.
      [1913 Webster]

   Folio post, a flat writing paper, usually 17 by 24 inches.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Folio \Fol"io\, v. t.
   To put a serial number on each folio or page of (a book); to
   page.
   [1913 Webster]

WordNet (r) 3.0 (2006):

folio
    n 1: the system of numbering pages [syn: pagination, folio,
         page number, paging]
    2: a sheet of any written or printed material (especially in a
       manuscript or book) [syn: leaf, folio]
    3: a book (or manuscript) consisting of large sheets of paper
       folded in the middle to make two leaves or four pages; "the
       first folio of Shakespeare's plays"