Extinction coefficient refers to several different measures of the absorption of light in a medium: Attenuation coefficient, sometimes called "extinction coefficient" in meteorology or climatology Mass extinction coefficient, how strongly a substance absorbs light at a given wavelength, per mass density Molar extinction coefficient, how strongly a substance absorbs light at a given wavelength, per molar concentration Imaginary part of the complex index of refraction, in physics See also For the quantitative relationship between the chemistry and physics definitions, see Mathematical descriptions of opacity de:Extinktionskoeffizient fr:Absorptivité molaire