Amplitude and waveform.

This is a graph of the way sound pressure changes over time for a simple sound:

The curve represents both positive and negative swings around the average pressure. The amount the curve deviates from this average is the amplitude. We hear amplitude of sounds as loudness.

The average pressure of air is around 15 pounds per square inch. Sound varies this by a thousandth or less.

The shape of the curve is very important in establishing the timbre of the sound. The shape of a sound is its waveform.