folio

noun

1. A leaf of a book or manuscript.
2. A page of a book, that is, one side of a leaf of a book.

   “Folio Page.—A page which occupies the half of a full sheet of paper, as Post-folio, Demy-folio, etc. Two pages of folio are imposed together as one form, four pages being a perfect sheet. Post-folio and Foolscap-folio, however, are more frequently imposed as four-page forms, and printed on Double-post and Double-foolscap paper.”

3. (broadly)A page number. The even folios are on the left-hand pages and the odd folios on the right-hand pages.

   “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.”

   “verso left-hand page of a book usually identified with even folio numbers. […] recto right-hand page of a book usually identified with odd folio numbers.”

4. (broadly)A sheet of paper folded in half.

   “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.”

5. (broadly)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 centimetres in height.

   “A rare copy of Shakespeare’s First Folio.”

   “A treatiſe in Folio, intitled, The Magazen of Honour, or a Treatiſe concerninge the Nobillitie of England accordinge to the Lawes of England, with their Priviledges in Parliament, & in what caſes noe priviledge, together with the Etymologie, definition & antiquitie of 3 ſortes of Barons.”

   “The imputed virtue of folios full of knockdown arguments is supposed to reside in them, just as some of the majesty of the British Empire dwells in the constable’s truncheon.”

   “Another, with the bump of order unnaturally developed, had his folios and quartos all reduced, in binding, to one size, so that they might look even on his bookshelves.”

   “He [collector Anthony Morris Storer] bequeathed part of his collection to […] Eton College, including the First Folio [of William Shakespeare], a fine copy of the Second Folio, a poor copy of the Third Folio, and the collection of Quartos.”

6. A wrapper for loose papers.
7. A page in an account book; sometimes, two opposite pages bearing the same serial number.
8. A protective case with a flap that folds to cover the screen of a mobile device.
9. (dated)A leaf containing a certain number of words; hence, a certain number of words in a piece of writing, as in England, in law proceedings 72, and in chancery, 90; in New York, 100 words.

   “If the office copy of a Will or any part of a Will or other document is required to be made fac-simile, and such Will or part of a Will or other document is two folios of ninety words in length or under, in addition to the fee for the copy … … … … [£]0 1[s.] 0[d.] / If exceeding two folios of ninety words, for every additional folio or part of a folio … … [£]0 0[s.] 6[d.]”

verb

1. (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.”

   “Folioing. The rule previously stated as to affidavits and other papers in respect to folioing applies also to petitions. They must be fairly and legibly written and indorsed, and where they are of more than two folios in length must be folioed.”

   “Folioing is necessary as an indication how far in his work the poster has proceeded, and must be done in both books immediately after each entry.”