Definify.com
Webster 1913 Edition
-mo
-mo
.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, sixteen
. It is taken from the Latin forms similarly used; mo
or 16mo
; eighteenmo or 18mo
as, duodecimo, sextodecimo, etc.
A small circle, placed after the number and near its top, is often used for -mo; as, 16°, 18°, etc.