What is meant by ‘disproportionation’? Give two examples of disproportionation reaction in aqueous solution.

The reactions in which same substance gets oxidized as well as reduced due to unstable oxidation state. 

Example: 

2MnO42- + 4H+ 2MnO4- + MnO2 + 2H2O

Mn (VI) is oxidised to Mn(VII) and also reduced to Mn(IV).  

2CrO43- +2H+ CrO42- +Cr3+ + 4H2O

Cr(V) is oxidised to Cr(VI) and also reduced to Cr(III).