noun, plural twen·ty·mos.
a book size of about 3 × 5 inches (8 × 12 cm), determined by printing on sheets folded to form 20 leaves or 40 pages.
a book of this size. Symbol: 20mo, 20°
Origin of twentymo
Also called vigesimo.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019