In SI units, absorption coefficient is measured in inverse metres, and is represented by the Greek letter α.
In chemistry, biological sciences and physics, the absorption coefficient is a measure of the how strongly a specific molecule will absorb energy and describes the propagation of sound or light through a specific material. It is very closely related to the absorption cross section.
Absorption coefficient is sufficient for describing energy propagation through a homogeneous system only. Propagation through a heterogeneous system is affected by scattering,. There is more general term attenuation that takes into account both absorption and scattering. It is widely used in acoustics as attenuation coefficient for characterizing particle size distribution. ,.