Amplitude

The AP Physics 2 CED puts it flatly at EK 14.1.A.6: amplitude is the maximum displacement of a wave from its equilibrium position. Symbol $A$. For a spring oscillator or a wave on a string that is a distance in meters; for a sound wave, EK 14.1.A.6.i also allows a pressure amplitude, the maximum increase or decrease in pressure from equilibrium pressure.

The mark this term costs. Amplitude does not set the period. AP Physics 1 EK 7.3.A.2 states that changing the amplitude of a system exhibiting SHM will not change the period of that system, and AP Physics C: Mechanics repeats it at 7.3.A.5. Pull the object further and it travels a longer path, but it also moves faster, and the two effects cancel.

What amplitude does change is energy. For a spring and object, $E_{\text{total}} = \frac{1}{2}kA^2$ (EK 7.4.A.4.ii), so doubling $A$ quadruples the total energy. For a wave, EK 14.1.A.6.iii says the energy carried increases with increasing amplitude and 14.1.A.6.ii says the loudness of a sound increases with it. Amplitude is independent of period and frequency (EK 14.2.A.1.iii), so a graph can pair any amplitude with any frequency.

Reading $A$ off a graph, and the proportional reasoning that goes with it, belong to the simple harmonic motion guide.