Definitions for "Kirchhoff's law"
A law that states: good absorbers of a given wavelength of radiation are also good emitters of that wavelength.
In radiation transfer, a law stating that in thermodynamic equilibrium and at a given wavelength the ratio of the intensity of emission to the absorptivity of any substance does not depend on the nature of the substance but rather only on the temperature and the wavelength, i.e. Another way of saying this is the the absorptivity and emissivity of a substance are equal and any single wavelength.
The radiation law which states that, at a given temperature, the ratio of the emissivity to the absorptivity for a given wavelength is the same for all bodies; this ratio is equal to the emissivity of an ideal blackbody at that temperature and wavelength. This law asserts that good absorbers of a given wavelength are also good emitters of the wavelength.