A figure of speech related to hyperbole that emphasizes the inexpressibility of some thing, idea, or feeling either by stating that words cannot describe it, or by comparing it with something (e.g., the heavens, the oceans) the dimensions of which cannot be grasped. Loosely related to the idea of the mathematical sublime. Examples abound — I can’t even begin to list just how often this type of hyperbolic rhetoric is thrown around.
Related: infandum, “the inexpressible,” which I’ll add an entry for later.
Also related: aposiopesis, also deserves an entry.
Somewhat related: paralepsis, apophasis, praeteritio/preterition, cataphasis (κατάφασις), antiphrasis (ἀντίφρασις), or parasiopesis (παρασιώπησις).
Citation: Oxford Dictionary of Literary Terms.