D defs.my
Entry 1 sense Webster, 1913

-mo

· IPA
01 A suffix added to the names of certain numerals or to the numerals themselves, to indicate the number of leaves made by folding a sheet of …
  1. 1.
    A suffix added to the names of certain numerals or to the numerals themselves, to indicate the number of leaves made by folding a sheet of paper; as, sixteenmo or 16mo; eighteenmo or 18mo. It is taken from the Latin forms similarly used; as, duodecimo, sextodecimo, etc. A small circle, placed after the number and near its top, is often used for -mo; as, 16°, 18°, etc.