octavo

noun, plural oc·ta·vos for 2.

1. a book size of about 6 × 9 inches (16 × 23 cm), determined by printing on sheets folded to form 8 leaves or 16 pages. Symbol: 8vo, 8°
2. a book of this size.

adjective

3. in octavo.

noun plural -vos

1. Also called: eightvo a book size resulting from folding a sheet of paper of a specified size to form eight leavesdemi-octavo Often written: 8vo, 8°
2. a book of this size
3. (formerly) a size of cut paper 8 inches by 5 inches (20.3 cm by 12.7 cm)

n.1580s, printer’s word for sheets folded to make eight leaves, from Latin in octavo “in the eighth,” ablative of octavus “eighth” (see octave). Abbreviation is 8vo.