folio
/ˈfəʊlɪəʊ/noun
(pl. -os)
1- an individual leaf of paper or parchment, either loose as one of a series or forming part of a bound volume, which is numbered on the recto or front side only（仅在正面标号的）一张纸; 一张羊皮纸书稿。
1.1
- the page number in a printed book（书的）页码。
1.2
- a sheet of paper folded once to form two leaves (four pages) of a book对折纸, 对开纸。
1.3
- a size of book made up of such sheets对开本：
imperial folio
皇家对开本
copies in folio.
对开本书册。
1.4
- a book or manuscript made up of sheets of paper folded in such a way; a volume of the largest standard size对开本书（或抄本）; 最大号对开本：
old vellum-bound folios
古老的犊皮纸包边的对开本
as modifier a folio volume.一本对开本。
词源
late Middle English: from Latin, ablative of folium 'leaf', in medieval Latin used in references to mean 'on leaf so-and-so'. The original sense of in folio （from Italian in foglio） was 'in the form of a full-sized sheet or leaf folded once' (designating the largest size of book).