The degree of reaction of an axial flow turbine is the ratio of isentropic temperature drop in a blade row to the

The degree of reaction of an axial flow turbine is the ratio of isentropic temperature drop in a blade row to the Total temperature drop in the stage.
The degree of reaction contributes to the stage efficiency and thus used as a design parameter. Stages having 50% degree of reaction are used where the pressure drop is equally shared by the stator and the rotor for a turbine.