The motion of two or more waves passing through the same medium at the same time is called?

The principle of superposition may be applied to waves whenever two (or more) waves travelling through the same medium at the same time.
 
The waves pass through each other without being disturbed. The net displacement of the medium at any point in space or time, is simply the sum of the individual wave displacements. This is true of waves which are finite in length (wave pulses) or which are continuous sine waves.