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"

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "folio":
   acknowledgments, article, back, back matter, bastard title,
   best seller, bibliography, book, book end, book support,
   book table, book tray, book truck, bookcase, bookholder, bookrack,
   bookrest, bookshelf, bookstack, bookstand, bound book, catch line,
   catchword, chapter, classic, clause, colophon, coloring book,
   contents, contents page, copyright page, crown, dedication,
   definitive work, duodecimo, eighteenmo, endleaf, endpaper,
   endsheet, errata, fascicle, flyleaf, folder, fore edge, foreword,
   front matter, gathering, great work, half-title page, hardback,
   head, imperial, imprint, index, inscription, introduction,
   juvenile, juvenile book, leaf, limp-cover book, magnum opus,
   makeup, medium, nonbook, notebook, novel, number, octavo,
   octodecimo, opus, opuscule, opusculum, page, paperback, paragraph,
   passage, phrase, picture book, playbook, pocket book, portfolio,
   prayer book, preface, preliminaries, production, psalmbook,
   psalter, publication, quarto, recto, reverso, revolving bookcase,
   royal, running title, section, sentence, serial, sextodecimo,
   sheet, signature, sixteenmo, sketchbook, soft-cover, songbook,
   stack, standard work, storybook, subtitle, super,
   table of contents, tail, text, title, title page, tome, trade book,
   trim size, twelvemo, type page, verse, verso, volume, work,
   writing