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

A reaction in which the same species is simultaneously oxidised as well as reduced is called disproportionation reaction.
When a particular oxidation state becomes less stable relative to other oxidation states, one lower, one higher it is said to undergo disproportionation. For example, manganese (VI) becomes unstable relative to manganese(Vll) and manganese(IV) in acidic solution.
3Mn^VI O₄^2– + 4H⁺ → 2Mn^VIII O₄^– + Mn^IV O₂ + 2H₂O