Source : Webster's Revised Unabridged Dictionary (1913)

Dodecagon \Do*dec"a*gon\, n. [Gr. ? twelve + ? angle: cf. F.
   dod['e]cagone.] (Geom.)
   A figure or polygon bounded by twelve sides and containing
   twelve angles.

Source : WordNet®

dodecagon
     n : a twelve-sided polygon