The term gray body (or graybody or grey body) is used in physics to mean an ideal body that reflects (i.e., does not absorb) a certain fraction of incident EMR, this same fraction applying to any wavelength, and by Kirchhoff's law of thermal radiation, emits black-body radiation at that wavelength by that same fraction of the EMR that a black body would emit, i.e., has it also the same emissivity at each wavelength. As such, its total EMR and its wavelength distribution matches that of a smaller black body and for an object too far to be resolved, it cannot be distinguished from a black body by the spectrum, but if it is close enough to be resolved, the EMR you see from a unit of visible area is less than from a black body.
Gray bodies and black bodies are ideal models. There is always reduced emissivity at some wavelengths (perhaps all wavelengths), which will not be same for every wavelength. An object might approximate a black body or gray body, and the ideal could offer a useful approximation of such an object.