Definition 3.4.1. Pyramid.
A pyramid is a polyhedron which may have any polygon as its base. All remaining faces are triangles that come together at a common vertex called the apex. The height of the pyramid is measured from the apex to the plane containing the base along a line perpendicular to this plane.
We name each pyramid according to its base. Thus, a triangular pyramid has a triangular base, a square pyramid has a square base, and so forth. Also, the apex of the pyramid does not have to be directly above the center of the base. In fact, the apex may not be directly above any part of the pyramid as shown in Figure 3.4.3.