in octavo
/ˌɪn ɒkˈteɪvəʊ/ /ˌɪn ɒkˈtɑːvəʊ/adverb
With leaves one-eighth of a standard printing sheet in size; in an edition having leaves of this size.
Origin
Late 16th century; earliest use found in Robert Parsons (1546–1610), Jesuit. From post-classical Latin in octavo in an eighth (sheet) from classical Latin in in + octāvo, ablative of octāvus eighth.