What does "folio" mean?

Additional senses

1. 2.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.
2. 3.(Print.) The page number. The even folios are on the left-hand pages and the odd folios on the right-hand.
3. 4.A page of a book; (Bookkeeping) a page in an account book; sometimes, two opposite pages bearing the same serial number.
4. 5.(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.
5. 6.To put a serial number on each folio or page of (a book); to page.
6. 7.Formed of sheets each folded once, making two leaves, or four pages; as, a folio volume. See Folio, n., 3.