What is a measure of the distance between a wave's crest and trough called?

The measure of the distance between a wave's crest and trough is called amplitude. Amplitude specifically refers to the maximum displacement of points on a wave, which in the case of a transverse wave, is the height from the middle of the wave (the equilibrium position) to either the crest (the highest point) or the trough (the lowest point). This characteristic is crucial in determining the wave's energy; higher amplitude indicates more energy in the wave.

In contrast, frequency refers to how many times the wave oscillates in a given time period, velocity indicates how fast the wave travels through space, and wavelength measures the distance between two successive crests or troughs. Each of these terms describes different properties of waves, but for measuring the distance between the crest and trough, amplitude is the appropriate concept.