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A gray atmosphere is the term for a model of an atmosphere for the purposes of radiative transfer models and similar uses, that does not model the differences in opacity present at different wavelengths. It is a simplification used in complex models (e.g., stellar structure) to help keep the entire model from becoming unwieldy. Typically, an approximation of the Rosseland mean opacity is used.
Other simplifications are often used with it to make stellar structure modeling tractable, examples being local thermodynamic equilibrium (LTE), a plane-parallel atmosphere and the Eddington approximation. The term gray atmosphere is sometimes used with the intent of including all these.