Even though most sounds have complex behavior and highly varying degrees of periodicity, it is sometimes helpful to simplify things, when talking about sound, by considering strictly periodic sounds.  Such a strictly periodic sound is one in which there is no variation in its oscillation and which exhibits the simplest spectral characteristics (we return to sound spectrum later on).  Such a sound is often referred to as a "pure tone."  Pure tones can be produced with a tuning fork or they can be produced electronically.

The following diagram depicts part of a pure tone.  Note the utter regularity of its pattern of troughs and crests.This is exhibited by the repeated pattern of up/down movements of the wave of motion (shown in purple). Once again, the lower part (the trough) corresponds to a lowering of air pressure while the higher part (the crest) corresponds to higher air pressure.

The x-axis constitutes elapsing time, while the y-axis measures the height of the crest.  The "standard" refers to undisplaced air pressure, or "equilibrium."  The height of the crest determines the amplitude of the waveform.  The velocity at which a waveform oscilates--i.e. the rate at which crests and troughs alternate--deterimines the frequency of the waveform.