The Collaborative International Dictionary of English v.0.48:

Undecagon \Un*dec"a*gon\, n. [L. undecim eleven + Gr. ? an
   angle.] (Geom.)
   A figure having eleven angles and eleven sides.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undecagon
    n 1: an eleven-sided polygon