Quarto

n.; pl. QUAR'TŌS. [Lat., from quartus, the fourth, from quatuor, four; Fr. & It. (in) quarto, Sp. (en) cuarto.] Originally, a book of the size of the fourth of a sheet of printing-paper; a size made by twice folding a sheet, which then makes four leaves; in present usage, a book of a square or nearly square form, corresponding to that formerly made by folding a sheet twice.