A gray atmosphere is the term for a model of an atmosphere regarding its opacity that ignores the differences in opacity at different wavelengths. It is a simplification used when incorporating opacity in more complex models (e.g., stellar structure) in order to keep the entire model from becoming unwieldy. Typically it uses a determination of the Rosseland mean opacity.
Other approximations in addition to the Rosseland Mean are commonly used with it to make stellar structure modeling tractable, examples being local thermodynamic equilibrium, a plane-parallel atmosphere and the Eddington approximation. The term gray atmosphere is sometimes used with the intent of including all these.