The term Black Body (or Blackbody or BB) is used in physics to mean an ideal body that reflects absolutely no light (or any Electromagnetic Radiation (EMR)), absorbing it all, i.e., is perfectly black, the only radiation from it is that it emits due to its Temperature. Internally, it is virtually at Thermodynamic Equilibrium (TE) if it has any significant lifetime as an EMR source, and to remain that way, the amount of incoming radiation must match the radiation it emits. The concept is useful as a first approximation of objects that are similar in that they emit significant radiation due to their temperature. The spectrum of the radiation of a black body (Black-Body Radiation) is characteristic, known as a Black Body Spectrum, and is an approximation of the spectra of stars, the Sun, planets, etc. Of interest is matching a body's spectrum with the black body temperature that would produce a close-to-matching spectrum, which helps determine the temperature of the body, i.e., detect the body's temperature with a Spectrograph.
Cosmic Microwave Background (CMB)
Effective Temperature (Teff)
Stefan-Boltzmann Constant (σ)
Stellar Temperature Determination
Sunyaev-Zel'dovich Effect (SZ Effect)
Thermodynamic Equilibrium (TE)
Wien's Displacement Law