- any of a group of substances that when added to water containing calcium and magnesium ions cause the ions to precipitate or change their usual properties: used in the purification of water for the laboratory, and for giving water more efficient sudsing ability with soap.
- any substance that lessens the hardness of water, usually by precipitating or absorbing calcium and magnesium ions
- a tank, apparatus, or chemical plant that is used to filter or treat water to remove chemicals that cause hardness
