The term black body in Physics is an ideal object that absorbs electromagnetic radiation at all wavelengths, with no reflection or transmission. Black body emits electromagnetic radiation relative only to its own temperature, according to Planck law. This radiation is called black body radiation. Radiation emitted is not affected by the radiation absorbed by the blackbody, except it's effect on the blackbody's own temperature.
A cavity blackbody is an object absorbs all the radiation that hits it regardless to the angle of incidence or wavelength. It is different from an extended area blackbody in the geometry and construction of the blackbody. Its cavity construction assures an emissivity close to a theoretical of an ideal blackbody of 1.
A measure of the ability of a surface to radiate energy; the ratio of the radiant flux emitted per unit area to that emitted by a black body at the same temperature. Symbol ε
The radiant flux, ϕ, is defined as power, or the energy per unit time, that is radiated from a source. It is measured in Watts or Joules per Sec.Radiant Intensity
Radiant intensity is defined as the Radiant flux power per unit of solid angle in a given direction by a source point. It is measured in Watts per steradian.
Thermal equilibrium occurs when the energy absorbed equals the energy emitted at a specified temperature for a given object. This follows Kirchhoff’s law of thermal radiation.