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°. [ 1913 Webster ]

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 ]