the.com/folio

means A sheet of paper folded once to make two leaves (four pages), or more broadly, a large-format book made of such sheets, or a single numbered leaf in a manuscript.

from From Latin folium, 'leaf' — the same root that gives us foliage and leaflet. The phrase in folio, 'in leaf form,' described a large sheet folded just once (as opposed to folding it into quarto or octavo for smaller books), and over time the fold-count name folio detached itself and became a noun of its own — the paper remembering, even now, that it used to be a leaf on a tree.