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