Black Body -

The Black Body: The Ideal Light That Revolutionized Physics A is a theoretical object that absorbs all incoming electromagnetic radiation that hits it, regardless of frequency or angle. Because it absorbs everything, it reflects and transmits nothing, making it a "perfect" absorber. While it sounds like a literal void, a black body is also the most efficient emitter of energy possible. How It Works: Absorption and Emission

The term was first defined by Gustav Kirchhoff in 1860. He realized that for an object to stay in thermal equilibrium, it must emit energy at the same rate it absorbs it. Blackbody Radiator - an overview | ScienceDirect Topics