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From 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]