Q. What do you mean by Wage inflation?

We will develop the Keynesian model removing the assumption of fixed nominal wages. We state wage inflation p_w as the percentage average increase in wages. Wage inflation and wages are still exogenous which means they aren't determined within the model. One explanation for this assumption is that wages frequently are determined by agreements which often last for many years. 

We don't need a new model to deal with inflation. Non-constant wages may be handled within all three Keynesian models as long as they are exogenous. Reason we chose to let wages be constant in previous Keynesian models were completely pedagogical - these models are easier to understand when wages are constant.