The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Bot.) Having four prominent longitudinal angles.
      [1913 Webster]

   3. (Crystallog.) Designating, or belonging to, a certain
      system of crystallization; dimetric. See Tetragonal
      system, under Crystallization.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tetragonal
    adj 1: of or relating to or shaped like a quadrilateral