A nine-sided shape is called a nonagon or enneagon. A nonagon is a type of polygon. Polygons are closed-plane figures that are made of line segments.
The name "nonagon" is derived from the combination of the Latin word "nonus," which means "ninth," and the Greek word "gonu," or "knee." The name "enneagon," derived wholly from Greek, is less common. The literal translation is "nine corners." The nonagon is an equilateral polygon. All of the lines are the same length, and the shape is convex. A straight line can be drawn through the shape, and it crosses two sides. Every interior angle is less than 180 degrees.