The Collaborative International Dictionary of English v.0.48:

deionizer \de*i"on*iz`er\ n. (Chem.)
   an apparatus used to remove ions from a solution; a common
   variant contains a mixture of cation exchange resin in the
   acid form and anion exchange resin in the hydroxyl form
   inside a replaceable cartridge; ions in aqueous solution are
   exchanged for the elements of water by passing the solution
   through the mixed resin.

   Note: In chemistry and biochemistry, experiments often
         require water free from contaminating ions, and
         ordinary distilled water may be further purified by
         passing through a deionizer in order to render the ion
         content sufficiently low so as not to interfere with
         the investigation.
         [PJC]