A Nonagon is 2-D shape and a polygon.
Like other polygons it is a closed figure. A nonagon is not a common polygon! In fact a nonagon has 9 sides, 9 angles and 9 vertices (corners).
Here are links to a cute musical video to help kids remember the name of this uncommon polygon plus the lyrics to the song. Enjoy!
~ TMBG: Here Comes the 123s Nonagon
~ Nonagon Lyrics