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.
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...