Lexitechnia

Cicero, Ad Atticum 7.13:

Ænigma […] plane non intellexi; est enim numero Platonis obscurius.

I didn’t quite understand your riddle; it’s more obscure than Plato’s number.

I’m not sure if ‘obscure’ is the best word to describe a riddle.

“Plato’s number,” or the nuptial number, is given as either 6³ or 60⁴, both of which are understood as being described in a complicated passage of his Republic (book 8, 545–547).

[For numerus.]