a. Having twenty-four leaves to a sheet; as, a twenty-fourmo form, book, leaf, size, etc. -- n. A book composed of sheets, each of which is folded into twenty-four leaves; hence, indicating more or less definitely a size of book whose sheets are so folded; -- usually written 24mo, or 24°. [ 1913 Webster ]
[にじゅうごし, nijuugoshi] (n) (See 二十四史) Twenty-Five Histories; authoritative collection of twenty-five Chinese historical books (the Twenty-Four Histories and the New History of Yuan)