A "black body" is a body which has the property of allowing all incident rays to enter without surface reflection and not allowing them to leave again.
Blackbodies are idealized surfaces that act as the perfect absorber and emitter. They serve as the standard against which real surfaces are compared when characterizing thermal radiation. A blackbody is defined by three characteristics:
# A blackbody absorbs all incident radiation, regardless of wavelength and direction.
# No surface can emit more energy than a blackbody for a given temperature and wavelength.
# A blackbody is a diffuse emitter.