Definition of "Hardness" :

noun: excessive sternness

noun: the quality of being difficult to do

"He assigned a series of problems of increasing hardness."

noun: devoid of passion or feeling; hardheartedness

noun: a quality of water that contains dissolved mineral salts that prevent soap from lathering

"The costs of reducing hardness depend on the relative amounts of calcium and magnesium compounds that are present."

noun: the property of being rigid and resistant to pressure; not easily scratched; measured on Mohs scale