water softener

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

• noun A substance used to reduce the hardness of water.
• noun A device that monitors and reduces the hardness of the water.

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

• noun A device which reduces the calcium and magnesium concentration of hard water by passing it through an ion exchange resin.

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

• noun a substance (such as sodium chloride) that lessens the hardness of water by replacing calcium and magnesium ions with sodium ions and so gives the water more efficient sudsing power