Architecture 101 : 5.

5. Solid-void theory is the three-dimensional counterpart to figure-ground theory. It holds that the volumetric spaces shaped or implied by the placement of solid objects are as important as, or more important than, the objects themselves.

A three-dimensional space is considered a positive space if it has a defined shape and a sense of boundary or threshold between in and out. Positive spaces can be defined in an infinite number of ways by points, lines, planes, solid volumes, tress, building edges, columns, walls, sloped earth, and innumerable other elements.

From 101 Things I learned in Architecture School by Matthew Frederick.