from The American Heritage® Dictionary of the English Language, 4th Edition
- n. The size (5 by 7 3/4 inches) of book pages formed by folding single sheets from a printing press into 12 leaves each.
- n. A book composed of pages of this size.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A size of paper, so called because it is originally made by folding and cutting a single sheet from a printing press into 12 leaves; (5 by 7¾ inches): 6.5 to 7.5 inches high, approximately 4.5 inches wide.
- n. A sheet or page of that size
- n. A book having pages of that size
from the GNU version of the Collaborative International Dictionary of English
- adj. Having twelve leaves to a sheet
- n. 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°.
from The Century Dictionary and Cyclopedia
- n. A size of page usually measuring, in the United States, about 5⅛ inches in width and 7⅔ inches in length, when the leaf is uncut, and corresponding to crown octavo of British publishers.
- n. A book composed of sheets which, when folded, form twelve leaves of this size.
- n. In music, the interval of a twelfth.
- Consisting of sheets folded into twelve leaves; having leaves or pages measuring about 5⅛ by 7⅔ inches. Often written 12mo or 12°.
Latin (in) duodecimō, (in) a twelfth, ablative of duodecimus, twelfth; see duodecimal.(American Heritage® Dictionary of the English Language, Fourth Edition)
Latin ablative of duodecimus ("twelfth") (from duodecim 'twelve', from duo 'two' + decem 'ten') (Wiktionary)