Amplitude is a measurement of the vertical distance of the wave from the average. The wave axis is the average height of the wave over one cycle, and is usually considered to be zero. Heights above and below the average are given positive and negative values, respectively.
The wave shape shown below is called a sine wave. The maximum and minimum amplitudes of this sine wave are the heights measured from the wave axis (white line) to the top of the wave peak and bottom of the wave trough. Everywhere else along the wave, amplitudes are less than the maximum.
Watch the animation to see examples of amplitude.
Bigger ocean waves have larger wave heights or amplitude. The amplitude of typical ocean waves is between 2 and 8 feet.