The distance from the rest position to the wave's crest is called the amplitude of the wave.

Here's a breakdown:

* Rest position: This is the undisturbed level of the medium the wave travels through. Think of it as the "normal" position.

* Crest: The highest point of a wave.

* Amplitude: The maximum displacement of the medium from its rest position, either upwards to the crest or downwards to the trough.

So, the amplitude is the distance from the rest position to either the crest or the trough.