The Collaborative International Dictionary of English v.0.48:

Depolarize \De*po"lar*ize\, v. t. [imp. & p. p. Depolarized;
   p. pr. & vb. n. Depolarizing.] [Pref. de- + polarize: cf.
   F. d['e]polarizer.]
   1. (Opt.) To deprive of polarity; to reduce to an unpolarized
      condition.
      [1913 Webster]

   Note: This word has been inaccurately applied in optics to
         describe the effect of a polarizing medium, as a
         crystalline plate, in causing the reappearance of a
         ray, in consequence of a change in its plane of
         polarization, which previously to the change was
         intercepted by the analyzer.
         [1913 Webster]

   2. (Elec.) To free from polarization, as the negative plate
      of the voltaic battery.
      [1913 Webster]

WordNet (r) 3.0 (2006):

depolarize
    v 1: eliminate the polarization of [syn: depolarize,
         depolarise]