The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]