A characteristic of water, imparted by salts of calcium, magnesium and iron, such as bicarbonates, carbonates, sulphate, chlorides, and nitrates that cause curdling of soap, deposition of scale in boilers, damage in some industrial process, and sometimes objectionable taste.

It may be determined by a standard laboratory procedure for computed from the amounts of calcium and magnesium as well as iron, aluminum, manganese, barium, strontium and zinc; expressed as equivalent parts per million of calcium carbonate.