Folio has a number of meanings. Throughout this site I concentrate on just one meaning ~ a single sheet of paper that has been folded ~ just once ~ to form a mini book ~ of four pages ~ two leaves ~ one sheet .
There are other meanings ~ mentioned below. Hence the need for me to avoid confusion by describing the paragraph above. My 'folded sheet' descriptor, above, does not have any other simple, one word, to explain it. That is why I am clarifiying my use of it throughout the rest of this site.
The folio is the main element to the whole history of codex-style bookbinding .
Other meanings attached to the word folio are normally clear from the context, but they can also lead to misunderstandings ~ always something to be avoided.
A folio is understood by some people to mean a single sheet of paper, or a leaf. For some the leaf has to be numbered in order to quaify as a folio.
A folio is sometimes interpreted as a large book. (As in Shakespeare's plays. A quote from a past Christie's Auction Catalogue lists the following item ~ 'The First Folio # 1623 ~ contains 36 plays, 18 of which had not previously been printed, and which would otherwise have been lost forever.' Their estimate of the value of a similar book was £1million.)
Sometimes a folio might refer to one half of a sheet once it has been cut into two pieces.
The word portfolio is sometimes shortened to folio, because of the folded V-shape of the covers.
I believe a folio can also mean a bundle of accounts or a particular legal document ~ the characteristic being that just one number refers to the whole thing ~ which maybe has four pages ~ or not. (As, maybe, 'see folio 23ff'.) I am not prepared to get entangled with either the accountancy or legal professions, but would as always, be glad to share any knowledgeable and reliable information.