Source : Webster's Revised Unabridged Dictionary (1913)

Duodecimo \Du*o*dec"i*mo\, n.; pl. {Duodecimos}.
   A book consisting of sheets each of which is folded into
   twelve leaves; hence, indicating, more or less definitely, a
   size of a book; -- usually written 12mo or 12[deg].

Duodecimo \Du`o*dec"i*mo\, a. [L. in duodecimo in twelfth, fr.
   duodecimus twelfth, fr. duodecim twelve. See {Dozen}.]
   Having twelve leaves to a sheet; as, a duodecimo from, book,
   leaf, size, etc.