octavo
Concise Oxford English Dictionary © 2008 Oxford University Press:
octavo /ɒkˈtɑːvəʊ, -ˈteɪ-/ (abbrev.: 8vo)
▶noun (pl. octavos) a size of book page that results from folding each printed sheet into eight leaves (sixteen pages).
– origin C16: from L. in octavo ‘in an eighth’, from octavus ‘eighth’.
'octavo' also found in these Oxford entries:

