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Amplitude is the measure of the height of a [[wave]].
[[Amplitude]] is the measure of the strength of a [[wave]].
Since there are many types of wave, their amplitude has to be measured in different ways.
An abstract [[sine wave]] on a sheet of graph paper can be characterized by its maximum height above the ''x'' axis. The simplest, <math>y = sin (x)</math>, has an amplitude of +/- 1. The class of "real" waves that are sine-like can be similarly measured, for instance, water waves on the [[ocean]] or a [[lake]]. It is important to differentiate between measuring the displacement from rest versus the peak-to-peak distance.
Sound waves can be expressed as a difference in pressure between their peak [[compression]]s and [[rarefaction]]s, or, more practically, by their loudness or ''[[sound pressure level]]''.
An [[earthquake]] is (usually) a longitudinal wave in the surface of the earth, and while the motion back and forth can be measured, their strength is usually expressed on the [[Richter Scale]].