n. (pl. -os) Printing 1 the size given by folding a (usu. specified) sheet of paper twice.
2 a book consisting of sheets folded in this way.
Usage:
Abbr.: 4to.
Phrases and idioms:
quarto paper paper folded in this way and cut into sheets.
Etymology: L (in) quarto (in) the fourth (of a sheet), ablat. of quartus fourth