University of Georgia
University of Georgia: Inter Math: Nonagon
This site gives the general definition of a nonagon with examples of a regular (convex) and irregular concave nonagons. There are links to related terms, challenging problems, more information, and the interactive check point involving...
National Council of Teachers of Mathematics
The Math Forum: Plane Figure
This site provides a definition for plane figure (at the top of the page) and a few examples.