A body is in radiative equilibrium when absorbed radiative power equals emitted radiative power. A surface absorbing flux and emitting as a grey body has equilibrium temperature .
A material element is in local radiative equilibrium when its absorbed and emitted radiative powers balance, so radiation causes no net local heating. In a non-scattering grey atmosphere in local thermodynamic equilibrium, this reduces to equality of the angular mean specific intensity and the Planck function . Integrating the radiative transfer equation over solid angle then shows that the radiative flux is constant through a plane-parallel atmosphere. The radiation can remain anisotropic; equilibrium constrains its angular mean, not every ray separately.
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Radiative equilibrium refers to a state in which the energy absorbed by an object or system from incoming radiation is equal to the energy it emits as radiation. In this state, there is no net gain or loss of energy, and the temperature of the object or system remains constant over time. In the context of planetary atmospheres, radiative equilibrium is particularly important for understanding the balance between incoming solar radiation (energy received from the Sun) and outgoing thermal radiation (energy emitted back into space).