/ok'teuh des"euh moh'/ , n. , pl. octodecimos for 2, adj.
n.
1. a book size of about 4 × 6 1 / 4 in. (10 × 16 cm), determined by printing on sheets folded to form 18 leaves or 36 pages. Symbol: 18mo, 18°
2. a book of this size.
adj.
3. in octodecimo. Also called eighteenmo .
[ 1855-60; short for NL in octodecimo in an eighteenth (of a sheet) ]