ion exchange

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A reversible chemical reaction between an insoluble solid and a solution during which ions may be interchanged, used in water softening and in the separation of radioactive isotopes.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry the reversible process whereby ions are exchanged between a solution and a solid or gel; the process used in a water softener

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a process in which ions are exchanged between a solution and an insoluble (usually resinous) solid; widely used in industrial processing