A solid region is any finite three dimensional region. The boundary of this region, a closed surface, is the surface the separates the interior of the solid region from everything else. So long as the region is finite (ie. does not go to infinity in any direction) the boundary is closed. Note that the terms used to describe an object give a clue as to whether the object is the closed surface or the interior.
| solid region | surface |
|---|---|
| ball | sphere, spherical shell |
| solid cylinder | tube, tube with endcaps |
| box, solid cube | cube (with six faces) |