What do we call the maximum displacement of a wave from its rest position?

The maximum displacement of a wave from its rest position is referred to as amplitude. This term specifically describes how far the wave's peaks (crests) and valleys (troughs) move away from the equilibrium or rest position. Amplitude is a critical characteristic of waves because it is directly related to the energy carried by the wave; larger amplitudes correspond to waves that carry more energy.

In contrast, other terms such as wavelength describe the distance between consecutive crests or troughs, frequency relates to how many cycles of the wave occur in a given time period, and period is the time it takes to complete one full cycle of the wave. Thus, while each of these other terms plays an important role in understanding wave behavior, they do not signify the maximum displacement from the rest position, making amplitude the correct answer.