Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

decagon
     n : a polygon with 10 sides and 10 angles