Hardness
"A water-quality indication of the concentration of alkaline salts in water, mainly calcium and magnesium. If the water you use is "hard" then more soap, detergent or shampoo is necessary to raise a lather." (U.S. Geological Survey, 2019)
Sources
U.S. Geological Survey (USGS). "Water Science Glossary Terms". USGS Water Science School. (2019). Link: https://water.usgs.gov/edu/dictionary.html#W