Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. A sheet of paper once folded.

   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}.

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

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

   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.

   {Folio post}, a flat writing paper, usually 17 by 24 inches.