**A nine-sided polygon is called a nonagon.** The prefix "non" is derived from the Latin "nonus," which means "ninth" to allude to the nine sides and angles of the polygon.

Another less common name for the nonagon is "enneagon." The name is derived from the Greek word "enneagonon," where "ennea" means "nine," and "gonon" means "corner." Thus, "enneagonon" means "nine corners," which is an accurate description of this polygon. Nonagons can be divided into regular and irregular nonagons. A regular nonagon has nine equal sides and nine equal internal angles of 140 degrees. An irregular nonagon has at least one side either longer or shorter than the rest.