What is 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 16º called?

sextodecimo — 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 16º. (Webster's 1913.)