The removal of calcium and magnesium, the ions which are the principle cause of hardness, from water.
Compare with water softener and water hardness. Removal of Ca2+ and Mg2+ from water to prevent undesirable precipitation reactions from occurring in plumbing, pools, washwater, and boilers.
To soften water fully, the minerals (calcium and magnesium) that cause hardness must be removed. These minerals are removed by ion-exchange.