From Longman Dictionary of Contemporary Englishwater softenerˈwater ˌsoftener noun 1 [uncountable]TIHC a chemical used for removing unwanted minerals from water2 [countable]THC a piece of equipment used for removing unwanted minerals from water
Examples from the Corpus
water softener• The second is to install a water softener into the cold water supply.• A water softener must be fitted after the supply to the drinking water so that this is left providing hard water for drinking.• Benckiser is best known for its Calgon water softener. 