vigesimo-quarto, 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]
vigesimo-quarto, 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 so made; -- usually written 24mo, or 24. [1913 Webster]