Webster Dictionary

1. Vigesimo-quarto(adj)

   having twenty-four leaves to a sheet; as, a vigesimo-quarto form, book, leaf, size, etc

2. Vigesimo-quarto(noun)

   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¡.