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The term gray body (or graybody or grey body) is used in physics to refer to an ideal body that absorbs a certain fraction of any incident electromagnetic radiation (EMR), such that this same fraction applies to each wavelength. By Kirchhoff's law of thermal radiation, it emits thermal radiation at each wavelength by that this same fraction of the EMR that a black body would emit, i.e., has it also the same emissivity at each wavelength. Any incident EMR that is not absorbed is reflected. The body's total emission and its emitted wavelength-distribution matches that of a smaller black body and for an object too far to be resolved (and presuming no EMR to reflect), it cannot be distinguished from a black body by the spectrum. If it is close enough to be resolved, the EMR you see from a unit of visible area (specific intensity) is less than that of a black body.
Both 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 may approximate a black body or gray body, and these ideals sometimes offer useful approximations of such objects.