Dictionary.net

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.

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.

A book having the sheet folded into four leaves and nearly square in size.

Having the sheet folded into four leaves.

A book of a quarto size:-pl. QUARTOS.

A book so folded.

Having four leaves to the sheet.

A book or pamphlet whose pages are of the size of the fourth of a sheet.

n. [Latin] A book of the size of the fourth of a sheet of printing paper, or in which every sheet being doubled makes four leaves ; also, the size of a sheet or book in which the paper is folded to make four leaves ;-written 4to.