The Collaborative International Dictionary of English v.0.48:

Vigesimo-quarto \Vi*ges"i*mo-quar"to\, n.; pl. -tos.
   A book composed of sheets each of which is folded into
   twenty-four leaves; hence, indicating more or less definitely
   a size of book so made; -- usually written 24mo, or 24[deg].
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Vigesimo-quarto \Vi*ges"i*mo-quar"to\, a. [L. vigesimus quartus
   twenty-fourth. Cf. Duodecimo.]
   Having twenty-four leaves to a sheet; as, a vigesimo-quarto
   form, book, leaf, size, etc.
   [1913 Webster]