The Collaborative International Dictionary of English v.0.48:

Polygon \Pol"y*gon\, n. [Gr. poly`gwnos polygonal; poly`s many +
   gwni`a angle: cf. F. polygone.] (Geom.)
   A plane figure having many angles, and consequently many
   sides; esp., one whose perimeter consists of more than four
   sides; any figure having many angles.
   [1913 Webster]

   Polygon of forces (Mech.), a polygonal figure, the sides of
      which, taken successively, represent, in length and
      direction, several forces acting simultaneously upon one
      point, so that the side necessary to complete the figure
      represents the resultant of those forces. Cf.
      Parallelogram of forces, under Parallelogram.
      [1913 Webster]

WordNet (r) 3.0 (2006):

polygon
    n 1: a closed plane figure bounded by straight sides [syn:
         polygon, polygonal shape]