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

Yes, that's true.  It's a term commonly used in 3D printing, to refer to models that have been created in compliance with 3D printing technology, which mostly require a solid, closed surfaces.  For decades, people were using techniques to save polygons by not modelling parts of models that would never be seen by a virtual camera, etc..  It's also a good strategy for today's physically accurate render engines, where the quality of the lighting, and realism can rely heavily on the realism of the objects in the scene.  Using manifold topology can help with complex physical light calculations, and calculating certain physical surface shaders, etc.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.