One condition for wave interference is that the sources of the waves must be coherent, which means they emit identical waves with a constant phase difference. I can understand that identical waves means they have the same wavelength. However, I don't understand what is a constant phase difference. The phase difference is always a constant, isn't it? But this definition clearly implies that it could be a non-constant. y=A*cos(wt + theta), where theta is the initial phase.