oc·ta·vo 
(ŏk-tāvō, -tä-)
(ŏk-tāvō, -tä-)n. pl. oc·ta·vos
1. The page size, from 5 by 8 inches to 6 by 9.5 inches, of a book composed of printer's sheets folded into eight leaves.
2. A book composed of octavo pages. In both senses also called eightvo.
[Medieval Latin (in) octāvō, (in) an eighth, from Latin, ablative sing. of octāvus, eighth, from octō, eight; see oktō(u) in the Appendix of Indo-European roots.]