## Geometry and Trigonometry## Solid Geometry |

## What is a polyhedron? |

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.”