quar·to 
(kwôrtō)
(kwôrtō)n. pl. quar·tos
1. The page size obtained by folding a whole sheet into four leaves.
2. A book composed of pages of this size.
[Short for Middle English (in) quarto, (in) the fourth part (of a sheet), from Medieval Latin (in) quārtō, from Latin, ablative of quārtus, fourth; see kwetwer- in the Appendix of Indo-European roots.]