Source : Webster's Revised Unabridged Dictionary (1913)

Octagon \Oc"ta*gon\, n. [Gr. ? eight-cornered; ? (for ? eight) +
   ? an angle: cf.F. cctogone.]
   1. (Geom.) A plane figure of eight sides and eight angles.

   2. Any structure (as a fortification) or place with eight
      sides or angles.

   {Regular octagon}, one in which the sides are all equal, and
      the angles also are all equal.

Source : WordNet®

octagon
     n : an eight-sided polygon