Hard water
Water's hardness is determined by the concentration of multivalent cations in the water. Multivalent cations are positively charged metal complexes with a charge greater than 1+. Usually, the cations have the charge of 2+. Common cations found in hard water include Ca and Mg. These ions enter a water supply by leaching from minerals within an aquifer. Common calcium-containing minerals are calcite and gypsum. A common magnesium mineral is dolomite (which also contains calcium). Rainwater and distilled water are soft, because they contain few ions.
Stiffness
The inverse of stiffness is "flexibility" or "compliance", typically measured in units of metres per Newton. In rheology, it may be defined as the ratio of strain to stress, and so take the units of reciprocal stress, "e.g". 1/Pa.