zone-planehttp://www.oed.com/dictionary/zone-plane_n%3Ftab%3Dmeaning_and_usezone-plane, n.
First published 1921; not fully revised

zone-planenoun

  1. 1878–
    1. 1878
      The plane to which they [sc. the edges of the crystal] are all perpendicular is called the zone plane.
      H. P. Gurney, Crystallography 22

Originally published as part of the entry for zone, n.

zone, n. was first published in 1921; not fully revised.