Pyramids

A pyramid (from Greek πυραμίδoς - pyramidos, genitive of πυραμίς - pyramis) is any three-dimensional polyhedron where the faces other than the base are triangular and converge on one point, called the apex. The base of a pyramid can be any polygon but is typically a square, leading to four non-base faces.

A pyramid is said to be regular if its base is a regular polygon and its upper faces are congruent isosceles triangles.

Cutting off the top of a pyramid, using a plane parallel to the plane of the base, leaves a frustum of a pyramid, sometimes called a flat-topped pyramid, though it no longer satisfies the definition of a pyramid.