What is the name for a figure with ten sides?

A figure with ten sides is called a decagon. The name combines the Greek words deka, meaning “ten,” and gonia, meaning “angle,” so it describes a polygon with ten sides and ten angles.

A regular decagon has ten equal sides and ten equal interior angles. Each interior angle measures 144 degrees, and the sum of all interior angles is 1,440 degrees. Like other regular polygons, it can be constructed from a circle; its geometry is closely connected to the golden ratio.

Decagons are sometimes confused with decahedra. A decagon is a flat, two-dimensional polygon, while a decahedron is a three-dimensional solid with ten faces. The prefixes are related, but “-gon” refers to angles or sides in a plane, whereas “-hedron” refers to faces on a solid.

The neighboring names follow the same pattern: an octagon has eight sides, a nonagon has nine, and a hexagon has six. A ten-sided solid die is often called a d10, but its three-dimensional shape is generally a pentagonal trapezohedron rather than a decagon.