1. An ideal emitter that radiates energy at the maximum possible rate per unit area at each wavelength for any given temperature. A blackbody also absorbs all the radiant energy in the near visible spectrum incident upon it. No actual substance behaves as a true blackbody, although platinum black and other soots rather closely approximate this ideal. However, one does speak of a blackbody with respect to a particular wavelength interval. This concept is fundamental to all the radiation laws, and is to be compared with the similarly idealised concepts of the whitebody and the graybody. In accordance with Kirchhoff's law, a blackbody not only absorbs all wavelengths but emits at all wavelengths and does so with maximum possible intensity for any given temperature.

2. A laboratory device that simulates the characteristics of a blackbody. See blackbody radiator.