folio

1. A leaf of a book or manuscript
2. A page of a book, that is, one side of a leaf of a book.
3. A page number. The even folios are on the left-hand pages and the odd folios on the right-hand pages.
4. A sheet of paper folded in half.
5. (books) A book made of sheets of paper each folded in half (two leaves or four pages to the sheet); hence, a book of the largest kind, exceeding 30 cm in height.

   A rare copy of Shakespeare’s First Folio.

6. A page in an account book; sometimes, two opposite pages bearing the same serial number.
7. (19th to early 20th century) 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.
8. A wrapper for loose papers.

verb

To put a serial number on each folio or page of (a book); to page