What does "zonal" mean?

Zonal: Of or pertaining to a zone; having the form of a zone or zones. 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.