What describes the amplitude of a wave?

The amplitude of a wave is defined as the maximum displacement of a point on the wave from its equilibrium position. In other words, it measures how far the wave moves above or below its rest position. This characteristic of a wave is critical because it determines the energy carried by the wave; larger amplitudes typically correspond to waves that carry more energy.

In contrast, the distance between two crests describes the wavelength of the wave, which measures the length of one complete cycle of the wave. The speed of the wave refers to how fast the wave propagates through a medium and does not directly relate to amplitude. Lastly, the wavelength indicates the distance over which the wave's shape repeats and is also not related to amplitude. Thus, the choice that accurately captures the essence of amplitude is the maximum displacement from the equilibrium position.