The Collaborative International Dictionary of English v.0.48:

Ionization \I`on*i*za"tion\, n. (Elec. Chem.)
   the process of converting neutral atoms or molecules into
   ions. The process may occur by dissolving an ionic substance
   in a dissociating solvent, such as water, or by adding or
   subtracting an electron to or from an atom or molecule. The
   latter process occurs, for example, in an electron beam in a
   mass spectrometer and by interaction of substances with
   ionizing radiation.
   [PJC]

WordNet (r) 3.0 (2006):

ionization
    n 1: the condition of being dissociated into ions (as by heat or
         radiation or chemical reaction or electrical discharge);
         "the ionization of a gas" [syn: ionization, ionisation]
    2: the process of ionizing; the formation of ions by separating
       atoms or molecules or radicals or by adding or subtracting
       electrons from atoms by strong electric fields in a gas [syn:
       ionization, ionisation]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "ionization":
   alpha decay, anion, atom-chipping, atom-smashing,
   atomic disintegration, atomic reaction, atomization, beta decay,
   bombardment, breeding, bullet, cation, chain reaction, cleavage,
   disintegration series, dissociation, electrocoating,
   electroetching, electrogalvanization, electrogilding,
   electrograving, electrolysis, electrolyte, electroplating,
   exchange reaction, fission, fission reaction, galvanization,
   gamma decay, ion, ionogen, neutron reaction, nonelectrolyte,
   nonreversible reaction, nuclear fission, nucleization,
   photodisintegration, proton gun, proton reaction,
   reversible reaction, splitting the atom, stimulation, target,
   thermonuclear reaction