In statistical mechanics, the excitation temperature (Tex) is defined for a population of particles via the Boltzmann factor.
It satisfies where Thus the excitation temperature is the temperature at which we would expect to find a system with this ratio of level populations.
However it has no actual physical meaning except when in local thermodynamic equilibrium.
The excitation temperature can even be negative for a system with inverted levels (such as a maser).
In observations of the 21 cm line of hydrogen, the apparent value of the excitation temperature is often called the "spin temperature".