Phase Shift
Is defined as the change in angle between the same point in two waves (see here).
That is to say the smallest horizontal delta needed for a point travelling in one wave to reach the same amplitude in another wave. Or the amount you'd need to add (or subtract) from a waves angle before they perfectly overlap.
Let s1 = sin(x), s2 = sin(y) and pd = phase difference between s1 and s2.
sin(x - pd) = sin(y) for all x, y.
Note: phase is calculated to mod 360 degrees. If [see page 18, two waves] are an even number of cycles apart (phase=360) we treat the phase as 0 (360 mod 360 = 0 mod 360).