\Sex`to*dec"i*mo\, a. [L. sextus-decimus the
sixteenth; sextus the sixth (fr. sex six) + decimus the
tenth, from decem ten. See {-mo}.]
Having sixteen leaves to a sheet; of, or equal to, the size
of one fold of a sheet of printing paper when folded so as to
make sixteen leaves, or thirty-two pages; as, a sextodecimo
volume.

\Sex`to*dec"imo\, n.; pl. {Sextodecimos}.
A book composed of sheets each of which is folded into
sixteen leaves; hence, indicating, more or less definitely, a
size of a book; -- usually written 16mo, or 16[deg].