hardness

1. The degree of firmness of a solid, as determined by its resistance to deformation, scratching, or abrasion. SEE ALSO: h. scale, number. 2. The relative penetrating power of a beam of x-rays, used both within the diagnostic range of energy and in radiation therapy; expressed in terms of half-value layer.

- indentation h. a number related to the size of the impression made by an indenter (or tool) of specific size and shape under a known load.

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hard·ness (hahrdґnis) 1. a quality of water produced by soluble salts of calcium and magnesium or other substances which form an insoluble curd 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. See also hardness number, under number. 3. the quality of x-rays that determines their penetrating power; hardness depends on wavelength: the shorter the wavelength, the harder the rays and the greater their penetrating power. 4. the degree of refraction of the residual gas in a glass tube: the higher the vacuum, the shorter the wavelength of the resulting x-rays.