The Collaborative International Dictionary of English v.0.48:

Zonal \Zon"al\, a. [L. zonalis.]
   Of or pertaining to a zone; having the form of a zone or
   zones.
   [1913 Webster]

   Zonal equation (Crystallog.), the mathematical relation
      which belongs to all the planes of a zone, and expresses
      their common position with reference to the axes.

   Zonal structure (Crystallog.), a structure characterized by
      the arrangements of color, inclusions, etc., of a crystal
      in parallel or concentric layers, which usually follow the
      outline of the crystal, and mark the changes that have
      taken place during its growth.

   Zonal symmetry. (Biol.) See the Note under Symmetry.
      [1913 Webster]