From AMS Glossary
A fundamental radiation law that equates the absorptivity of matter to its emissivity at the same wavelength.
Loosely put, this important law asserts that good absorbers of radiation at a given wavelength are also good emitters at that wavelength. For Kirchhoff's law to hold, the matter must be in local thermodynamic equilibrium, but the law is otherwise quite general, and applies to both natural and idealized surfaces or volumes. For natural surfaces it is often necessary to make the absorptivities and emissivities functions of direction and polarization state before applying the law.