zone-levellinghttp://www.oed.com/dictionary/zone-levelling_n%3Ftab%3Dmeaning_and_usezone levelling, n.
First published 1986; not fully revised

zone levellingnoun

  1. 1953–
    A process similar to zone refining in which the molten zone is passed repeatedly to and fro to produce a more homogeneous material.
    1. 1953
      The discussion of zone-melting has been confined to the categories of zone-leveling and zone-refining.
      W. G. Pfann in Transactions Amer. Inst. Mining & Metall. Engineers vol. CXCIV. 752/1
    2. 1978
      A modification of the technique, zone levelling, is used to introduce controlled amounts of impurity (for example, indium into germanium).
      P. W. Atkins, Physical Chemistry x. 301

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

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