1- A substance that is used produce soft water (water with reduced hardness). 2- A device used for the same purpose.
A water softener converts hard water, through ion exchange, into soft water. Soft water is mainly used in closed piping systems, such as heating and air-conditioning systems, to inhibit the formation of scale. Hard water, or simply tap water, is not to be confused with heavy water, which is mainly used in nuclear reactors!