The Collaborative International Dictionary of English v.0.48:

Decagon \Dec"a*gon\, n. [Pref. deca- + Gr. ? a corner or angle:
   cf. F. d['e]cagone.] (Geom.)
   A plane figure having ten sides and ten angles; any figure
   having ten angles. A regular decagon is one that has all its
   sides and angles equal.
   [1913 Webster]

WordNet (r) 3.0 (2006):

decagon
    n 1: a polygon with 10 sides and 10 angles