What is a 10-sided polygon called?

A polygon with 10 sides is called a decagon. The word comes from the Greek words for "ten" ("deka") and for "angle" or "corner" ("gonia"). Decagons have 10 angles and 10 sides.

The five-pointed star on the flag of the United States is actually an irregular decagon, as each of the stars has 10 sides. It is considered irregular because the angles and sides are not equal. A regular decagon, a decagon whose 10 sides are all equal in length, has interior angles that each contain 144 degrees. The exterior angles, or the angles formed by the outside corners of a decagon, each contain 36 degrees.