Achaea is the term that refers, quite frankly, to everything. It encompasses not only the vast and multifarious planes of existence that constitute the multiverse, but also the seas of shifting, unpredictable chaos outside and between them.
There was once much contention as to the shape of Achaea. Fortunately, the argument was settled by Sarapis when He proclaimed that Achaea was "created in imitation of an n+1 Calabi-Yau space."