noun

• (by extension, printing)
  "Folio.—The running number of the pages of a work. When there is no running title, the folio is placed in the centre of the head-line; when there is a running title, at the outside corner—the even folio on the left, the odd on the right. The preface, contents, index, and all introductory matter, usually have separate folios inserted in Roman lower-case numerals."
• (by extension, printing)
  "This pocket copying-machine, by which letters and writings of all sizes, even folio, may be very quickly copied, consists of a press four inches long, with a small cylinder, which contains pens and ink: the whole apparatus may be conveniently carried in the pocket, and the price is very moderate."
• (by extension, printing)
  "A rare copy of Shakespeare’s First Folio."
• (accounting) A page in an account book; sometimes, two opposite pages bearing the same serial number.
  "In the leger,^([sic]) articles of the ſame kind are collected together; and, for that purpoſe, it is divided into many accounts, under which the different branches of buſineſs are arranged. Each account is introduced by a proper title, to explain the nature of the articles it contains; and articles of oppoſite kinds, which belong to the ſame account, are placed on the oppoſite pages of the ſame folio: for inſtance, money received on the one ſide, and money paid on the other; or goods bought on the one ſide, and goods ſold on the other."

verb

• (transitive) To put a serial number on (a folio or page, or on all the folios or pages of a book); to foliate, to page.
  "And all the pleadings and other proceedings and copies thereof shall be fairly and legibly written; and if not so written, and folioed, and indorsed as aforesaid, the clerks shall not file such as may be offered to them for that purpose, nor will the court hear any motion or application founded thereon."