The Collaborative International Dictionary of English v.0.48:

ionize \ionize\ v. i.
   1. (Physics, Chemistry) to dissociate into ions, as by
      dissolution in water or another polar solvent.
      [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

ionize \i"on*ize\, v. t. [imp. & p. p. Ionized; p. pr. & vb.
   n. Ionizing.] (Physics, Chemistry)
   To convert (an atom or molecule) into ions, e. g. by
   dissolving in water, by reaction with acid or alkali, or by
   interaction with ionizing radiation. See ionization.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

ionize
    v 1: become converted into ions [syn: ionize, ionise]
    2: convert into ions [syn: ionize, ionise]