1. The electromagnetic radiation emitted by an ideal blackbody; it is the theoretical maximum amount of radiant energy of all wavelengths that can be emitted by a body at a given temperature. The spectral distribution of blackbody radiation is described by Planck law and the related radiation laws. If a tiny opening is made into an otherwise completely enclosed space (hohlraum), the radiation passing out through this hole when the walls of the enclosure have come to thermal equilibrium at some temperature will closely approximate ideal blackbody radiation for that temperature.

2. Any physical body absorbs and emits electromagnetic radiation when its temperature is above absolute zero. Planck's law determines the radiant flux of a body at a specific wavelength. In atmospheric chemistry, the calculation involving Earth's blackbody radiation shows that Earth's surface temperature would be below the freezing point of water if it did not have an atmosphere that absorbed some of the outgoing radiation.