"Pyramid is a three-dimensional figure or space figure that has triangles as its faces.
There are two kinds of pyramid: triangular pyramid and square pyramid.
Triangular pyramid has a triangle as its base and its faces. So any face of the pyramid can be the base. Square pyramid has a square as its base and triangles as its faces. Only the square can be the base. Thus, square pyramid is the space figure that has only one base and triangles as its faces.