Blackbody radiation" or "cavity radiation" refers to an object or system which absorbs all radiation incident upon it and re-radiates energy which is characteristic of this radiating system only, not dependent upon the type of radiation which is incident upon it. The radiated energy can be considered to be produced by standing wave or resonant modes of the cavity which is radiating.
2.2 Absorptance, and the Definition of a Black Body.
If a body is irradiated with radiation of wavelength l, and a fraction a(l) of that radiation is
absorbed, the remainder being either reflected or transmitted, a(l) is called the absorptance at
wavelength l . Note that l is written in parentheses, to mean "at wavelength l", not as a
subscript, which would mean "per unit wavelength interval". The fractions of the radiation
reflected and transmitted are, respectively, the reflectance and the transmittance. The sum of the
absorptance, reflectance and transmittance is unity, unless you can think of anything else that
might happen to the radiation.
A body for which a(l) = 1 for all wavelengths is a black body.
A body for which a has the same value for all wavelengths, but less than unity, is a grey body