Boundary

A boundary is "a line that marks the limit of an area; a dividing line"(-Webster's dictionary.) In architecture this may be the exterior wall of a building. For instance there was not much of a boundary between the temple of Hatshepsut and the mountain in which it was built. The mountain and the temple morphed into one space because of the way the temple was designed. With the pyramids at Giza there are many boundary lines made by the numbers of pyramids all in one space. One boundary, for instance, delineates the outside, or the inside of the group of pyramids. You can walk around the outside of the group of pyramids or you can cross that boundary to the interior and look at them from the center of their gathering. 

The drawing below is of an "Urban studio wall." I chose to include this to be an example of a boundary. Urban studio walls delineate between the interior and the exterior.