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, y = sin(x), 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.