In chemistry, the molar absorption coefficient or molar attenuation coefficient () is a measurement of how strongly a chemical species absorbs, and thereby attenuates, light at a given wavelength. It is an intrinsic property of the species. The SI unit of molar absorption coefficient is the square metre per mole (), but in practice, quantities are usually expressed in terms of ⋅cm or L⋅mol⋅cm (the latter two units are both equal to ). In older literature, the cm/mol is sometimes used; 1 M⋅cm equals 1000 cm/mol. The molar absorption coefficient is also known as the molar extinction coefficient  and molar absorptivity, but the use of these alternative terms has been discouraged by the IUPAC.