\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.

\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].