A book (or manuscript) consisting of large sheets of paper folded in the middle to make two leaves or four pages WordNet 3.0 & Open English WordNet

The term "folio" has three interconnected but distinct meanings in the world of books and printing: first, it is a term for a common method of arranging sheets of paper into book form, folding the sheet only once, and a term for a book made in this way; second, it is a general term for a sheet, leaf or page in manuscripts and old books; and third, it is an approximate term for the size of a book, and for a book of this size. Wikipedia

A book size, the page number of books, or sheets with multiple printed pages Wikipedia Disambiguation

A sheet of paper folded once that constitutes two leaves or four pages of a book or manuscript. OmegaWiki

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 cm in height. Wiktionary