= Solution
A <blackbody> is an ideal body that absorbs all incident <electromagnetic radiation>, at every <frequency> and direction. Its <absorptivity> is one, so <Kirchhoff's law of thermal radiation> makes its equilibrium <emissivity> one as well. At a specified <temperature> its thermal <spectrum> is universal: it depends on <temperature>, not material or shape. This does not mean it looks black when hot; it can glow brightly while remaining a perfect absorber.
A small hole in a nearly isothermal opaque cavity approximates a <blackbody>, because radiation entering the hole undergoes many interactions before it could escape. In <thermal equilibrium> the cavity contains an approximately isotropic <Planck spectrum>. The emitted <radiance> follows <Planck's law>, its integrated outward <radiant exitance> follows the <Stefan–Boltzmann law>, and its wavelength peak follows <Wien's displacement law>. A real surface can have wavelength- and direction-dependent <emissivity> below this ideal limit.
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