What is meant by 'disproportionation' of an oxidation state? Give an example.

(N/A) reaction in which a species is simultaneously oxidized and reduced is called a disproportionation reaction. In this process,a particular oxidation state becomes less stable relative to other oxidation states,one lower and one higher.
For example,the manganate $(VI)$ ion $(MnO_4^{2-})$ is unstable in acidic solution and undergoes disproportionation to form permanganate $(VII)$ $(MnO_4^-)$ and manganese $(IV)$ oxide $(MnO_2)$.
$3MnO_4^{2-} + 4H^{+} \to 2MnO_4^{-} + MnO_2 + 2H_2O$