A nonagon, also called an enneagon, is a polygon with nine sides. The name comes partially from the Latin word for ninth, "nonus."
A nonagon is an uncommon polygon compared to other polygons like hexagons or octagons. A regular nonagon is a shape where all nine sides are the same length, which consequently means that all internal angles are equal at 140 degrees. Along with nine sides, a nonagon has nine vertices and 27 possible diagonals that can be drawn from vertex to vertex. The most closely related polygons are the eight-sided octagon, like a stop sign, and the 10-sided decagon.