A body that emits only a fraction of the thermal energy emitted by an equivalent blackbody. By definition, a graybody has a surface emissitivy less than 1, and a surface reflectivity greater than zero.
A thermal reference source which maintains an output, not at 100% emission (which would be a blackbody) but at a lesser ratio. Graybody emissions maintain a constant emission ratio over a wide frequency range. This distinguishes them from spectral radiators.