hardness /härd′nəs/ [AS, heard, hard] , 1. a quality of water produced by soluble salts of calcium and magnesium or other substances that form a precipitate with soap and thus interfere with its cleansing power. 2. the quality of firmness produced by cohesion of the particles composing a substance, as evidenced by its inflexibility or resistance to indentation, distortion, or scratching.
