Water having a low concentration of calcium and magnesium ions. According to U.S. Geological Survey guidelines, soft water is water having a hardness of 60 milligrams per liter or less.
Water with light concentrations of calcium and magnesium, and which lathers readily with soap or detergent.
water that contains mostly sodium or potassium ions. Hard water can be "softened" by replacing calcium and magnesium for sodium or potassium ions using a water softener system. Water naturally low in TDS is also called soft water.