quar·to \ˈkwȯr]d.(ˌ)ō, -ȯ(ə)], ](ˌ)tō\noun (-s) Etymology: Latin, abl. of quartus fourth 1.: the size of a piece of paper cut four from a sheet; also: paper or a page of this size — abbr. 4to; symbol 4°; see book tables 2.: a book printed on quarto pages < the volume is a quarto, bound on the narrow side — Walter Millis >