The Collaborative International Dictionary of English v.0.48:

Tetragon \Tet"ra*gon\, n. [L. tetragonum, Gr. tetra`gwnon;
   te`tra- (see Tetra-) + gwni`a corner, angle: cf. F.
   t['e]tragone.]
   1. (Geom.) A plane figure having four sides and angles; a
      quadrangle, as a square, a rhombus, etc.
      [1913 Webster]

   2. (Astrol.) An aspect of two planets with regard to the
      earth when they are distant from each other ninety
      degrees, or the fourth of a circle. --Hutton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tetragon
    n 1: a four-sided polygon [syn: quadrilateral, quadrangle,
         tetragon]