Source : Webster's Revised Unabridged Dictionary (1913)

Tetragonal \Te*trag"o*nal\, a.
   1. (Geom.) Of or pertaining to a tetragon; having four angles
      or sides; thus, the square, the parallelogram, the
      rhombus, and the trapezium are tetragonal fingers.

   2. (Bot.) Having four prominent longitudinal angles.

   3. (Crystallog.) Designating, or belonging to, a certain
      system of crystallization; dimetric. See {Tetragonal
      system}, under {Crystallization}.

Source : WordNet®

tetragonal
     adj : of or relating to or shaped like a quadrilateral