/ ɒkˈteɪvəʊ; -ˈtɑːv-; NAmE ɑːk-; -voʊ/ noun
( pl. -os ) ( technical ) a size of a book page that is made by folding each sheet of paper into eight leaves (= 16 pages)
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WORD ORIGIN
late 16th cent.: from Latin in octavo in an eighth, from octavus eighth.