The Collaborative International Dictionary of English v.0.48:

deionized \de*i"on*ized`\ a. (Chem.)
   freed from ions by a process of deionization; as, deionized
   water.

   Note: In chemistry and biochemistry, experiments often
         require water free from contaminating ions, and the
         ordinary distilled water may be subjected to a process
         of deionization in order to render the ion content
         sufficiently low so as not to interfere with the
         investigation.
         [PJC]