The word polyhedron comes from the Greek poly (meaning “many”) and the Indo-European word hedron (meaning “seat”). In geometry, a polyhedron is considered to be a three-dimensional solid that represents the union of polygonal regions, usually joined at their edges and having no gaps.

Polyhedrons are classified as convex, or if you extend any side, the figure lies on only one side of the plane (for example, a pyramid and cube); a concave polyhedron is one that can extend on both sides of such a plane (for example, a form that exhibits concavity on one or all sides of the polyhedron). The plural of polyhedron is often seen as “polyhedrons,” but it is more correct to use “polyhedra.”