From what I understand, if a gas has temperature $T$, it means that the probability of a gas particle to have energy $E$ is proportional to $\exp(-E/kT)$. It is the physical interpretation of temperature.

But it is only true when the gas is in Maxwell-Boltzmann Distribution. Electrons in plasma are in MB distribution. So when electron temperature is 1 eV for instance, I can know the energy distribution of electrons. 

However, ions in plasma can be not in MB distribution. So, when it says ion temperature is 0.02 eV for instance, I don't know what it represents. What does ion temperature measure?