A book of the largest size, formed by folding the paper only once, so that each sheet makes two leaves. It is from the Italian, un libro in foglio, through the French, in-folio. Fol. is the contraction for folio.
in mercantile books, means page so-and-so, and sometimes the two pages which lie exposed at the same time, one containing the credit and the other the debit of one and the same account. So called because ledgers, etc., are made in folio. The paging is called the folio also. Printers call a page of MS. or printed matter a folio regardless of size.
In conveyances seventy-two words, and in Parliamentary proceedings ninety words, make a folio.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894