Definition of hardness

• n. - The quality or state of being hard, literally or figuratively.
• n. - The cohesion of the particles on the surface of a body, determined by its capacity to scratch another, or be itself scratched;-measured among minerals on a scale of which diamond and talc form the extremes.
• n. - The peculiar quality exhibited by water which has mineral salts dissolved in it. Such water forms an insoluble compound with soap, and is hence unfit for washing purposes.