Source : Webster's Revised Unabridged Dictionary (1913)

Quarto \Quar"to\, a. [L. in quarto in fourth, from quartus the
   fourth: cf. F. (in) quarto. See {Quart}.]
   Having four leaves to the sheet; of the form or size of a
   quarto.

Quarto \Quar"to\, n.; pl. {Quartos}.
   Originally, a book of the size of the fourth of sheet of
   printing paper; a size leaves; in present usage, a book of a
   square or nearly square form, and usually of large size.

Source : WordNet®

quarto
     n : the size of a book whose pages are made by folding a sheet
         of paper twice to form four leaves [syn: {4to}]