Soft Water

any water which contains less than 1.0 gpg(17.1 mg/l) of hardness minerals, expressed as calcium carbonate.

Definitioner

mg/l: The abbreviation for milligrams per liter.
Hardness: a characteristic of natural water due to the presence of dissolved calcium and magnesium; water hardness is responsible for most scale formation in pipes and water heaters, and forms insoluble" curd" when it reacts with soaps. Hardness is usually expressed in grains per gallon, parts per million, or milligrams per liter, all as calcium carbonate equivalent.
GPG: abbreviation for 'grain per gallon'.
Carbonate: the Co3- ion.
Calcium: one of the principal elements making up the earths crust, the compounds of which make the water hard. The presence of calcium in water is a factor contributing to the formation of scale and insoluble soap curds which are a means of clearly identifying hard water.