The Collaborative International Dictionary of English v.0.48:

Hendecagon \Hen*dec"a*gon\, n. [Gr. ? eleven + ? angle: cf. F.
   hend['e]cagone.] (Geom.)
   A plane figure of eleven sides and eleven angles. [Written
   also endecagon.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Endecagon \En*dec"a*gon\, n. [See Hendecagon.] (Geom.)
   A plane figure of eleven sides and angles.
   [1913 Webster]