The Collaborative International Dictionary
Enneagon
Enneagon \En"ne*a*gon\ (?; 277), n. [Gr. 'enne`a nine + gwni`a corner, angle: cf. enn['e]agone.] (Geom.) A polygon or plane figure with nine sides and nine angles; a nonagon.
Wiktionary
enneagon
n. (context geometry English) A polygon with nine sides; a nonagon.