Quote - > Quote - Topologically closed.

That seems suitable, but there's another one.  Maybe the term is "manifold"?  :unsure:  Not that it's crucially important.  :lol:

That is the term I thought you were looking for.  One of the cooler aspects of Wings 3D, once you appreciates its usefulness, is that it is a manifold modeler.  Every operation of Wings is based around the idea that there is the space around the model, and there is another distinct space inside it.  That is why Wings by default produces (almost?) exclusively watertight models.  In order to create a one sided or "open" object in Wings, you practically have to use "export selected" on a set of faces.  And when these objects are imported back into Wings, the models are actually closed during conversion to a winged edge data structure.